hwmon: fix a NULL vs IS_ERR_OR_NULL() check in xgene_hwmon_probe()

The devm_memremap() function returns error pointers on error,
it doesn't return NULL.

Fixes: c7cefce0 ("hwmon: (xgene) access mailbox as RAM")
Signed-off-by: Xinghuo Chen <xinghuo.chen@foxmail.com>
Link: https://lore.kernel.org/r/tencent_9AD8E7683EC29CAC97496B44F3F865BA070A@qq.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>