serial: bfin_sport_uart: zero sport_uart_port if allocated dynamically

Need to initialize the SPORT state rather than using random memory.

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>