scsi: myrb: Fix up null pointer access on myrb_cleanup()

scsi: myrb: Fix up null pointer access on myrb_cleanup()

When myrb_probe() fails the callback might not be set, so we need to
validate the 'disable_intr' callback in myrb_cleanup() to not cause a null
pointer exception. And while at it do not call myrb_cleanup() if we cannot
enable the PCI device at all.

Link: https://lore.kernel.org/r/20220523120244.99515-1-hare@suse.de
Reported-by: Zheyu Ma <zheyuma97@gmail.com>
Tested-by: Zheyu Ma <zheyuma97@gmail.com>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>