phy: core: don't require set_mode() callback for phy_get_mode() to work

phy: core: don't require set_mode() callback for phy_get_mode() to work

As reported by Damon Ding, the phy_get_mode() call doesn't work as
expected unless the PHY driver has a .set_mode() call. This prompts PHY
drivers to have empty stubs for .set_mode() for the sake of being able
to get the mode.

Make .set_mode() callback truly optional and update PHY's mode even if
it there is none.

Cc: Damon Ding <damon.ding@rock-chips.com>
Link: https://lore.kernel.org/r/96f8310f-93f1-4bcb-8637-137e1159ff83@rock-chips.com
Tested-by: Damon Ding <damon.ding@rock-chips.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20250209-phy-fix-set-moe-v2-1-76e248503856@linaro.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>