spi: ti_qspi: Fix baudrate divider calculation

Fix the divider calculation logic to choose a value so that the
resulting baudrate is either equal to or closest possible baudrate less
than the requested value. While at that, cleanup ti_spi_set_speed().

Signed-off-by: Vignesh R <vigneshr@ti.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>