tty: serial/bcm63xx_uart: fix sparse warning

this patch fixes following sparse warnings:

bcm63xx_uart.c:857:43: warning: Using plain integer as NULL pointer
bcm63xx_uart.c:871:35: warning: Using plain integer as NULL pointer

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>