spi: spidev: fix a potential use-after-free in spidev_release()

[ Upstream commit 06096cc6c5a84ced929634b0d79376b94c65a4bd ]

If an spi device is unbounded from the driver before the release
process, there will be an NULL pointer reference when it's
referenced in spi_slave_abort().

Fix it by checking it's already freed before reference.

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@gmail.com>
Link: https://lore.kernel.org/r/20200618032125.4650-2-zhenzhong.duan@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>