m68k: amiserial - Kill warn_unused_result warnings

warning: ignoring return value of 'request_irq', declared with attribute
warn_unused_result

and clean up the error path handling.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Alan Cox <alan@lxorguk.ukuu.org.uk>