amiserial: expand "custom"

"custom" macro is a too generic name. Expand it -- that is use
amiga_custom on all the locations.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210714091314.8292-8-jslaby@suse.cz


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