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

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

[ Upstream commit 10fce7ebe888fa8c97eee7e317a47e7603e5e78d ]

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

Fixes: c7cefce03e69 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>