[SCSI] fnic: Fix SGEs limit

Driver allows IOs with more SGEs than max SGEs supported by Palo. The current
max SGEs supported by the fnic driver is 1024. The current register settings
on Palo supports a max of 256 only. Palo would return any IO with more than
256 SGEs with an error indicating INVALID_SGLS. Fnic driver should limit the
max supported SGLs in the driver to 256 to avoid this error.

Signed-off-by: Sesidhar Baddela <sebaddel@cisco.com>
Signed-off-by: Hiral Patel <hiralpat@cisco.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>