phy: ti: phy-omap-usb2: Fix NULL pointer dereference for SRP

If the external phy working together with phy-omap-usb2 does not implement
send_srp(), we may still attempt to call it. This can happen on an idle
Ethernet gadget triggering a wakeup for example:

configfs-gadget.g1 gadget.0: ECM Suspend
configfs-gadget.g1 gadget.0: Port suspended. Triggering wakeup
...
Unable to handle kernel NULL pointer dereference at virtual address
00000000 when execute
...
PC is at 0x0
LR is at musb_gadget_wakeup+0x1d4/0x254 [musb_hdrc]
...
musb_gadget_wakeup [musb_hdrc] from usb_gadget_wakeup+0x1c/0x3c [udc_core]
usb_gadget_wakeup [udc_core] from eth_start_xmit+0x3b0/0x3d4 [u_ether]
eth_start_xmit [u_ether] from dev_hard_start_xmit+0x94/0x24c
dev_hard_start_xmit from sch_direct_xmit+0x104/0x2e4
sch_direct_xmit from __dev_queue_xmit+0x334/0xd88
__dev_queue_xmit from arp_solicit+0xf0/0x268
arp_solicit from neigh_probe+0x54/0x7c
neigh_probe from __neigh_event_send+0x22c/0x47c
__neigh_event_send from neigh_resolve_output+0x14c/0x1c0
neigh_resolve_output from ip_finish_output2+0x1c8/0x628
ip_finish_output2 from ip_send_skb+0x40/0xd8
ip_send_skb from udp_send_skb+0x124/0x340
udp_send_skb from udp_sendmsg+0x780/0x984
udp_sendmsg from __sys_sendto+0xd8/0x158
__sys_sendto from ret_fast_syscall+0x0/0x58

Let's fix the issue by checking for send_srp() and set_vbus() before
calling them. For USB peripheral only cases these both could be NULL.

Fixes: 657b306a ("usb: phy: add a new driver for omap usb2 phy")
Signed-off-by: Tony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/r/20240128120556.8848-1-tony@atomide.comSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/phy/ti/phy-omap-usb2.c

@@ -117,7 +117,7 @@ static int omap_usb_set_vbus(struct usb_otg *otg, bool enabled)
 {
 	struct omap_usb *phy = phy_to_omapusb(otg->usb_phy);
 
-	if (!phy->comparator)
+	if (!phy->comparator || !phy->comparator->set_vbus)
 		return -ENODEV;
 
 	return phy->comparator->set_vbus(phy->comparator, enabled);
@@ -127,7 +127,7 @@ static int omap_usb_start_srp(struct usb_otg *otg)
 {
 	struct omap_usb *phy = phy_to_omapusb(otg->usb_phy);
 
-	if (!phy->comparator)
+	if (!phy->comparator || !phy->comparator->start_srp)
 		return -ENODEV;
 
 	return phy->comparator->start_srp(phy->comparator);