spi: efm32: add spi_bitbang_stop to device remove callback

This call is needed to cleanup the resources requested by
spi_bitbang_start in the probe callback.

Noticed-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Mark Brown <broonie@linaro.org>