[SCSI] qla4xxx: Fix srb lookup in qla4xxx_eh_device_reset

eh_device_reset may be called from scsi error handler or sg_reset, etc.
When called from sg_reset, there will not be an associated srb.  The
driver should lookup the corresponding device handle given information
from the supplied cmd structure and should not assume that there exists
an srb.

Signed-off-by: Karen Higgins <karen.higgins@qlogic.com>
Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>