[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>