serial: 8250_bcm7271: use NULL to initialized a null pointer

Pointer membase is currently being in initialized with zero rather
than NULL. Fix this.

Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20210719095533.14017-1-colin.king@canonical.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>