serial: bfin_sport_uart: add missing mapbase initialization

The driver doesn't care about this, but the common serial core wants it.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>