clk: shmobile: fix sparse NULL pointer warning

Fixes the following sparse warnings:

drivers/clk/shmobile/clk-sh73a0.c:57:17: warning:
 Using plain integer as NULL pointer

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>