ufs: add get_max_pwr_mode callback

ufs: add get_max_pwr_mode callback

Add a new get_max_pwr_mode callback to permit the UFS controller
driver manipulate the max_pwr_mode struct right before setting
the new pwr_mode to the UFS device.

It can be used to limit the HS Gear with errata and hardware
limitations on some UFS controllers.

Tested-by: Julius Lehmann <lehmanju@devpi.de>
Tested-by: Caleb Connolly <caleb.connolly@linaro.org> #rb3gen2
Link: https://lore.kernel.org/r/20240910-topic-ufs-qcom-controller-v1-2-54c0d2231b10@linaro.org
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>