omap2/3/4: serial: remove initialization sparse warnings

Initialization of pointer should be done with NULL. Removes sparse
warnings:
arch/arm/mach-omap2/serial.c:566:17: warning: Using plain integer as NULL pointer
arch/arm/mach-omap2/serial.c:567:17: warning: Using plain integer as NULL pointer

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>