net: broadcom CNIC: requires MMU

The CNIC kconfig symbol selects UIO and UIO depends on MMU.
Since 'select' does not follow dependency chains, add the same MMU
dependency to CNIC.

Quietens this kconfig warning:

WARNING: unmet direct dependencies detected for UIO
  Depends on [n]: MMU [=n]
  Selected by [m]:
  - CNIC [=m] && NETDEVICES [=y] && ETHERNET [=y] && NET_VENDOR_BROADCOM [=y] && PCI [=y] && (IPV6 [=m] || IPV6 [=m]=n)

Fixes: adfc5217 ("broadcom: Move the Broadcom drivers")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Cc: Rasesh Mody <rmody@marvell.com>
Cc: GR-Linux-NIC-Dev@marvell.com
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: netdev@vger.kernel.org
Link: https://lore.kernel.org/r/20201129070843.3859-1-rdunlap@infradead.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/broadcom/Kconfig

@@ -88,6 +88,7 @@ config BNX2
 config CNIC
 	tristate "QLogic CNIC support"
 	depends on PCI && (IPV6 || IPV6=n)
+	depends on MMU
 	select BNX2
 	select UIO
 	help