ARM: u300: fix return value check in __u300_init_boardpower()

In case of error, the function syscon_node_to_regmap() returns ERR_PTR()
and never returns NULL. The NULL test in the return value check should
be replaced with IS_ERR().

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>