[SCSI] st: fix gcc 4.4 warning

This patch fixes the GCC 4.4 warning reported by David Binderman and Sergey
Senozhatsky. The old version was working correctly but was not easy to read.

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