spi: rspi: Add spi_master_get() call to prevent use after free

In rspi_remove(), current code dereferences rspi after spi_unregister_master(),
thus add an extra spi_master_get() call is necessary to prevent use after free.

Current code already has an extra spi_master_put() call in rspi_remove(), so
this patch just adds a spi_master_get() call rather than a spi_master_get() with
spi_master_put() calls.

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