serial: ami: 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/tty/amiserial: section mismatch in reference: amiga_serial_driver+0x8 (section: .data) -> amiga_serial_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/043afcbc94ad90079301f3c7738136a7993a1748.1711748999.git.u.kleine-koenig@pengutronix.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>