In the preparation of adding support for new gears, 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, make it clear that this driver only supports symmetric gear
setting (hs_tx_gear == hs_rx_gear).
Reviewed-by: Andrew Halaney <ahalaney@redhat.com>
Tested-by: Andrew Halaney <ahalaney@redhat.com> # Qdrive3/sa8540p-ride
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>