[SCSI] pmcraid: redundant check in pmcraid_check_ioctl_buffer()

struct pmcraid_ioctl_header member buffer_length is unsigned, so this
check appears redundant.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Acked-by: Anil Ravindranath <anil_ravindranath@pmc-sierra.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>