[SCSI] libfc: fix hton24 macro to take expressions as args

hton24(p + 3, value) would fail to compile because
p + 3[0] is not a valid expression.

Went ahead and converted hton24 and ntoh24 to inline
functions, which is better because the parameters
are evalutated only once.

Signed-off-by: Joe Eykholt <jeykholt@cisco.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>