sparc32: Fix comparing pointer to 0 coccicheck warning

Fixes coccicheck warning:

/arch/sparc/mm/srmmu.c:354:42-43: WARNING comparing pointer to 0

Avoid pointer type value compared to 0.

Reported-by: Tosk Robot <tencent_os_robot@tencent.com>
Signed-off-by: Kaixu Xia <kaixuxia@tencent.com>
Signed-off-by: David S. Miller <davem@davemloft.net>