ncr5380: Fix NCR5380_select() EH checks and result handling

Add missing checks for EH abort during arbitration and selection.
Rework the handling of NCR5380_select() result to improve clarity.

Fixes: 707d62b3 ("ncr5380: Fix EH during arbitration and selection")
Tested-by: Michael Schmitz <schmitzmic@gmail.com>
Cc: <stable@vger.kernel.org> # 4.5
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>