net: ethernet: ti/cpsw-common.c: fix sparse warning

this patch fixes following sparse warning:

cpsw-common.c:23:5: warning: symbol 'cpsw_am33xx_cm_get_macid' was not declared. Should it be static?

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>