staging: fbtft: Fix error path in fbtft_driver_module_init()

staging: fbtft: Fix error path in fbtft_driver_module_init()

If registering the platform driver fails, the function must not return
without undoing the spi driver registration first.

Fixes: c296d5f9957c ("staging: fbtft: core support")
Link: https://lore.kernel.org/r/20220118181338.207943-1-u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20220123175201.34839-2-u.kleine-koenig@pengutronix.de
Signed-off-by: Mark Brown <broonie@kernel.org>