scsi: libfc: Fix enum-conversion warning

gcc -Wextra points out an assignment between two different enum types:

drivers/scsi/libfc/fc_exch.c: In function 'fc_exch_setup_hdr':
../drivers/scsi/libfc/fc_exch.c:275:26: warning: implicit conversion from 'enum fc_class' to 'enum fc_sof' [-Wenum-conversion]

This seems to be intentional, as the same numeric values are used here, so
shut up the warning by adding an explicit cast.

Link: https://lore.kernel.org/r/20201026214911.3892701-1-arnd@kernel.org
Fixes: 42e9a92 ("[SCSI] libfc: A modular Fibre Channel library")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/libfc/fc_exch.c

@@ -271,7 +271,7 @@ static void fc_exch_setup_hdr(struct fc_exch *ep, struct fc_frame *fp,
 
 	if (f_ctl & FC_FC_END_SEQ) {
 		fr_eof(fp) = FC_EOF_T;
-		if (fc_sof_needs_ack(ep->class))
+		if (fc_sof_needs_ack((enum fc_sof)ep->class))
 			fr_eof(fp) = FC_EOF_N;
 		/*
 		 * From F_CTL.