serial: bfin_sport_uart: Use resource size to fix off-by-one error

Use the resource_size function instead of manually calculating the
resource size. This actually fixes an off-by-one error.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>