[ARM] 3983/2: remove unused argument to __bug()

It appears that include/asm-arm/bug.h requires include/linux/stddef.h
for the definition of NULL. It seems that stddef.h was always included
indirectly in most cases, and that issue was properly fixed a while ago.

Then commit 5047f09b incorrectly reverted
change from commit ff10952a

 (bad dwmw2)
and the problem recently resurfaced.

Because the third argument to __bug() is never used anyway, RMK suggested
getting rid of it entirely instead of readding #include <linux/stddef.h>
which this patch does.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>