spi: bcm2835: Fix use-after-free on unbind

bcm2835_spi_remove() accesses the driver's private data after calling
spi_unregister_controller() even though that function releases the last
reference on the spi_controller 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: f8043872 ("spi: add driver for BCM2835")
Reported-by: Sascha Hauer <s.hauer@pengutronix.de>
Reported-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Cc: <stable@vger.kernel.org> # v3.10+: 123456789abc: spi: Introduce device-managed SPI controller allocation
Cc: <stable@vger.kernel.org> # v3.10+
Cc: Vladimir Oltean <olteanv@gmail.com>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/ad66e0a0ad96feb848814842ecf5b6a4539ef35c.1605121038.git.lukas@wunner.de


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