Documentation: make configfs example code simpler, clearer

If "p" is NULL then it will cause an oops when we pass it to
simple_strtoul().  In this case "p" can not be NULL so I removed the
check.  I also changed the check a little to make it more explicit that
we are testing whether p points to the NUL char.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>