NULL: keep NULL definition in stddef.h only ...

remove other local definition of NULL, use
  #include <linux/stddef.h>
instead.
I use this command to search NULL definition,
  grep -R "define\s*\<NULL\>"
hope there are no more definition of NULL.

from ISO/IEC 9899:TC3:
The macros are
	NULL
which expands to an implementation-defined null pointer constant;

Signed-off-by: Du Huanpeng <u74147@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

svn_rev_639