scsi: ufs: Extract devfreq registration

Failing to register with devfreq leaves hba->devfreq assigned, which
causes the error path to dereference the ERR_PTR(). Rather than bolting
on more conditionals, move the call of devm_devfreq_add_device() into
it's own function and only update hba->devfreq once it's successfully
registered.

The subsequent patch builds upon this to make UFS actually work again,
as it's been broken since f1d981ea ("PM / devfreq: Use the available
min/max frequency")

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>