scsi: ufs: ufs-qcom: Factor out the logic finding the HS Gear

In the preparation of adding support for new gears, let's move the
logic that finds the gear for each platform to a new function. This helps
with code readability and also allows the logic to be used in other places
of the driver in future.

While at it, let's make it clear that this driver only supports symmetric
gear setting (hs_tx_gear == hs_rx_gear).

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Andrew Halaney <ahalaney@redhat.com>