megaraid_sas: Support for max_io_size 1MB

Driver will expose max sge = 256 (earlier it was 64) if firmware
supports extended IO size (1M).

Signed-off-by: Sumit Saxena <sumit.saxena@avagotech.com>
Signed-off-by: Kashyap Desai <kashyap.desai@avagotech.com>
Reviewed-by: Martin Petersen <martin.petersen@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>