usb: make module xhci_hcd removable

Fixed regression. After commit 29e409f0 ("xhci: Allow xHCI drivers to
be built as separate modules") the module xhci_hcd became non-removable.
That behaviour is not expected and there're no notes about it in commit
message. The module should be removable as it blocks PM suspend/resume
functions (Debian Bug#666406).
Signed-off-by: Arthur Demchenkov <spinal.by@gmail.com>
Reviewed-by: Andrew Bresticker <abrestic@chromium.org>
Cc: <stable@vger.kernel.org> # v3.18+
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/xhci.c

@@ -5011,4 +5011,12 @@ static int __init xhci_hcd_init(void)
 	BUILD_BUG_ON(sizeof(struct xhci_run_regs) != (8+8*128)*32/8);
 	return 0;
 }
+
+/*
+ * If an init function is provided, an exit function must also be provided
+ * to allow module unload.
+ */
+static void __exit xhci_hcd_fini(void) { }
+
 module_init(xhci_hcd_init);
+module_exit(xhci_hcd_fini);