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>
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