spi: Remove one needless transfer speed fall back case

Falling back to maximum speed of the controller in case of SPI slave
maximum speed is not set is needless. It already defaults to maximum
speed of the controller since commit 052eb2d4 ("spi: core: Set
max_speed_hz of spi_device default to max_speed_hz of controller").

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>