[SCSI] megaraid_sas: frame count optimization

This patch removes duplicated code in frame calculation &  adds
megasas_get_frame_count() that also takes into account the number of frames
that can be contained in the Main frame.
FW uses the frame count to pull sufficient number of frames from host memory.

Signed-off-by: Sumant Patro <Sumant.Patro@lsil.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>