rswitch: Fix imbalance phy_power_off() calling

The phy_power_off() should not be called if phy_power_on() failed.
So, add a condition .power_count before calls phy_power_off().

Fixes: 5cb63092 ("net: renesas: rswitch: Add phy_power_{on,off}() calling")
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>