net: stmicro: fix a missing check of clk_prepare

[ Upstream commit f86a3b83833e7cfe558ca4d70b64ebc48903efec ]

clk_prepare() could fail, so let's check its status, and if it fails,
return its error code upstream.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>