ufs: refactor configuring power mode

Sometimes, the device shall report its maximum power and speed
capabilities, but we might not wish to configure it to use those
maximum capabilities.
This change adds support for the vendor specific host driver to
implement power change notify callback.

To enable configuring different power modes (number of lanes,
gear number and fast/slow modes) it is necessary to split the
configuration stage from the stage that reads the device max power mode.
In addition, it is not required to read the configuration more than
once, thus the configuration is stored after reading it once.

Signed-off-by: Dolev Raviv <draviv@codeaurora.org>
Signed-off-by: Yaniv Gardi <ygardi@codeaurora.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>