sfc: Move shared NIC code from falcon.c to new source file nic.c

sfc-y			+= efx.o falcon.o tx.o rx.o falcon_gmac.o \

sfc-y			+= efx.o nic.o falcon.o tx.o rx.o falcon_gmac.o \

			   falcon_xmac.o selftest.o ethtool.o qt202x_phy.o \

			   mdio_10g.o tenxpress.o falcon_boards.o

sfc-$(CONFIG_SFC_MTD)	+= mtd.o