[SCSI] ibmvscsi: remove unnecessary map_sg check

No need to check use_sg since sg_tablesize is set appropriately in the
scsi host template.

Brian King's patch (2a730937

) did this
cleanup but the data buffer accessors patch (written before the patch
and merged after it) restored the check.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>