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: 42e9a92f

 ("[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>