spi: efm32: Don't call kfree() after spi_master_put()

Calling kfree() to clean up the memory obtained from spi_alloc_master() is
wrong as this is done in spi_master_release() when spi_master->dev's refcount
reaches zero.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@linaro.org>