[SCSI] libfc: remove define of fc_seq_exch in fc_exch.c

The define for fc_seq_exch is unnecessary, since it also appears in scsi/libfc.h

Signed-off-by: Hillf Danton <dhillf@gmail.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>