spi: Implement spi_set_speed

This function is already defined in spi.h but no implementation of it
currently exists in the tree. The implementation is based on the static
function spi_set_speed_mode(). The function prototype is modified so
that an success or error condition can be returned to the caller.

Signed-off-by: Paul Barker <paul.barker@sancloud.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>