scsi: a4000t: Mark driver struct with __refdata to prevent section mismatch

scsi: a4000t: Mark driver struct with __refdata to prevent section mismatch

As described in the added code comment, a reference to .exit.text is ok for
drivers registered via module_platform_driver_probe(). Make this explicit
to prevent the following section mismatch warning

	WARNING: modpost: drivers/scsi/a4000t: section mismatch in reference: amiga_a4000t_scsi_driver+0x8 (section: .data) -> amiga_a4000t_scsi_remove (section: .exit.text)

that triggers on an allmodconfig W=1 build.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/743c3cfaf12b9f61f66afa5529ac126c856e4d11.1711746359.git.u.kleine-koenig@pengutronix.de
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>