[SCSI] advansys: Remove a check for an impossible condition

AscExeScsiQueue() has one caller, and it passes the address of a
variable; this cannot ever be NULL.  This is the only place that ever
sets ASCQ_ERR_SCSIQ_NULL_PTR, so delete that error code too, as well as
several other unused ASCQ_ERR codes.

Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>