spi: spi-sh: Fix use-after-free on unbind

spi_sh_remove() accesses the driver's private data after calling
spi_unregister_master() even though that function releases the last
reference on the spi_master and thereby frees the private data.

Fix by switching over to the new devm_spi_alloc_master() helper which
keeps the private data accessible until the driver has unbound.

Fixes: 680c1305

 ("spi/spi_sh: use spi_unregister_master instead of spi_master_put in remove path")
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Cc: <stable@vger.kernel.org> # v3.0+: 5e844cc3

: spi: Introduce device-managed SPI controller allocation
Cc: <stable@vger.kernel.org> # v3.0+
Cc: Axel Lin <axel.lin@ingics.com>
Link: https://lore.kernel.org/r/6d97628b536baf01d5e3e39db61108f84d44c8b2.1607286887.git.lukas@wunner.de
Signed-off-by: Mark Brown <broonie@kernel.org>