ARM: 9218/1: dma-mapping: fix pointer/integer warning

Fix the use of a pointer assignment from integer where false
is being used instead of NULL. Fix the following warning by
usign NULL:

arch/arm/mm/dma-mapping.c:712:52: warning: Using plain integer as NULL pointer

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>