scsi: bfa: fix type conversion warning

A regression fix introduced a harmless type mismatch warning:

drivers/scsi/bfa/bfad_bsg.c: In function 'bfad_im_bsg_vendor_request':
drivers/scsi/bfa/bfad_bsg.c:3137:35: error: initialization of 'struct bfad_im_port_s *' from 'long unsigned int' makes pointer from integer without a cast [-Werror=int-conversion]
  struct bfad_im_port_s *im_port = shost->hostdata[0];
                                   ^~~~~
drivers/scsi/bfa/bfad_bsg.c: In function 'bfad_im_bsg_els_ct_request':
drivers/scsi/bfa/bfad_bsg.c:3353:35: error: initialization of 'struct bfad_im_port_s *' from 'long unsigned int' makes pointer from integer without a cast [-Werror=int-conversion]
  struct bfad_im_port_s *im_port = shost->hostdata[0];

This changes the code back to shost_priv() once more, but encapsulates
it in an inline function to document the rather unusual way of
using the private data only as a pointer to the previously allocated
structure.

I did not try to get rid of the extra indirection level entirely,
which would have been rather invasive and required reworking the entire
initialization sequence.

Fixes: 45349821 ("scsi: bfa: fix access to bfad_im_port_s")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>