net: cadence_gem: check packet size in gem_recieve

While receiving packets in 'gem_receive' routine, if Frame Check
Sequence(FCS) is enabled, it copies the packet into a local
buffer without checking its size. Add check to validate packet
length against the buffer size to avoid buffer overflow.

Reported-by: Ling Liu <liuling-it@360.cn>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>
