scsi: qedf: Simplify s/g list mapping

When mapping the pages from a scatter/gather list from the SCSI layer we
only need to follow these rules:

 - Max SGEs for each I/O request is 256
 - No size limit on each SGE
 - No need to split OS provided SGEs to 4K before sending to firmware.
 - Slow SGE is applicable only when:
   - There are > 8 SGEs and any middle SGE is less than a page size (4K)

Make necessary changes so that driver follows these rules. Applicable only
for Write requests (not for Read requests). No need to check SGE address
alignment requirements (first, middle or last) before declaring slow SGE.

Signed-off-by: Chad Dupuis <cdupuis@marvell.com>
Signed-off-by: Saurav Kashyap <skashyap@marvell.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>