power/reset: at91: fix return value check in at91_reset_platform_probe()

In case of error, the function devm_ioremap() returns NULL
not ERR_PTR(). The IS_ERR() test in the return value check
should be replaced with NULL test.

Fixes: ecfe64d8 ("power: reset: Add AT91 reset driver")
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Sebastian Reichel <sre@kernel.org>