[SCSI] st: Remove bogus memset

Mike Christie noticed a bogus memset. It can be removed as dead code
since the number of bytes in the driver buffer in fixed block mode is
always a multiple of the tape block size.

Signed-off-by: Kai Mäkisara <Kai.Makisara@kolumbus.fi>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>