altos: Use new ao_spi_speed inline to set SPI speeds using spec'd frequencies

altos: Use new ao_spi_speed inline to set SPI speeds using spec'd frequencies

Instead of hand-adjusting SPI speeds for each driver and product based
on processor supported rates, create an inline function that takes the
maximum frequency supported by the device and maps that to a value
supported by the SoC. This ensures that we don't accidentally drive devices
faster than they want (as could happen with AO_SPI_SPEED_FAST), but do go
as fast as possible in every configuration. Using an inline function means
the value is computed at compile time.

Signed-off-by: Keith Packard <keithp@keithp.com>