[SCSI] scsi dh alua: fix group id masking

The buf[i] is a byte but we are only asking 4 bits off the
group_id. This patch has us take off a byte.

Signed-off-by: Ilgu Hong <ilgu.hong@promise.com>
Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>