[SCSI] fnic: remove redundant BUG_ONs and fix checks on unsigned

The shost sg tablesize is set to FNIC_MAX_SG_DESC_CNT and fnic uses
scsi_dma_map, so both BUG_ONs can be removed.

scsi_dma_map may return -ENOMEM, sg_count should be int to catch that.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>