sunxi: fix CONFIG_SUNXI_EMAC references

fixes CONFIG_SUNXI_EMAC references from drivers/net/Makefile and
include/configs/sunxi-common.h likely forgotten in:
	commit abc3e4df ("sunxi: Convert SUNXI_EMAC to Kconfig")

Signed-off-by: Artturi Alm <artturi.alm@gmail.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>