net/mlxbf_gige: Fix an IS_ERR() vs NULL bug in mlxbf_gige_mdio_probe

[ Upstream commit 4774db8d ]

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

Fixes: 3a1a274e

 ("mlxbf_gige: compute MDIO period based on i1clk")
Signed-off-by: Peng Wu <wupeng58@huawei.com>
Link: https://lore.kernel.org/r/20220923023640.116057-1-wupeng58@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>