e100: Fix napi_enable BUG_ON when recovering from EEH error

BUG_ON(!test_bit(NAPI_STATE_SCHED, &n->state)) was being hit
during e100 EEH recovery. The problem source was a napi_enable
call being made during e100_io_error_detected. Napi should remain
disabled after e100_down, and only be reenabled when the interface
is recovered.

This patch also updates e100_io_error_detected in order to make
it similar to the current versions of the error_detected callback
in drivers such as e1000e and ixgbe.

Signed-off-by: Andre Detsch <adetsch@br.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>