GRETH: handle frame error interrupts

Frame error interrupts must also be handled since the RX flag only indicates
successful reception, it is unlikely but the old code may lead to dead lock
if 128 error frames are recieved in a row.

Signed-off-by: Daniel Hellstrom <daniel@gaisler.com>
Signed-off-by: David S. Miller <davem@davemloft.net>