[SCSI] libfc: update mfs boundry checking

A previous commit changed the mfs checking to ensure the new
mfs is less or equal to the mfs supported by the FCF. This
doesn't work for BRDCM cards as they set an mfs of 2048 regardless
of whether the switch returns a larger mfs.

This patch validates the new mfs against the upper and lower spec
defined boundries for a FCoE mfs.

Signed-off-by: Vasu Dev <vasu.dev@intel.com>
Signed-off-by: Bhanu Prakash Gollapudi <bprakash@broadcom.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>