Return back the error value that we get from phy_read_mmd().

Fixes: c84786fa ("net: phy: marvell10g: read copper results from CSSR1")
Signed-off-by: Baruch Siach <baruch.siach@siklu.com>
Reviewed-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/f47cb031aeae873bb008ba35001607304a171a20.1650868058.git.baruch@tkos.co.ilSigned-off-by: Paolo Abeni <pabeni@redhat.com>