octeontx2-pf: Fix assigned error return value that is never used

Currently when the call to otx2_mbox_alloc_msg_cgx_mac_addr_update fails
the error return variable rc is being assigned -ENOMEM and does not
return early. rc is then re-assigned and the error case is not handled
correctly. Fix this by returning -ENOMEM rather than assigning rc.

Addresses-Coverity: ("Unused value")
Fixes: 79d2be38 ("octeontx2-pf: offload DMAC filters to CGX/RPM block")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>