drivers: atm, char fix integer as NULL pointer warnings

drivers/atm/nicstar.c:418:25: warning: Using plain integer as NULL pointer
drivers/char/drm/r128_cce.c:820:25: warning: Using plain integer as NULL pointer
drivers/char/tty_io.c:1183:10: warning: Using plain integer as NULL pointer

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>