gianfar: fix babbling rx error event bug

Gianfar interrupt handler uses IEVENT_ERR_MASK to check and handle errors.
Babbling RX error (IEVENT_BABR) should be included in IEVENT_ERROR_MASK.
Otherwise if BABR is raised, it never gets handled nor cleared, and an
interrupt storm results. This has been observed to happen on sending a
burst of ethernet frames to a gianfar based board.

Signed-off-by: Xiaotian Feng <xiaotian.feng@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>