spi: spi-qcom-qspi: Fix use-after-free on unbind

qcom_qspi_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: f79a158d

 ("spi: spi-qcom-qspi: Use OPP API to set clk/perf state")
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Cc: <stable@vger.kernel.org> # v5.9+: 5e844cc3

: spi: Introduce device-managed SPI controller allocation
Cc: <stable@vger.kernel.org> # v5.9+
Cc: Rajendra Nayak <rnayak@codeaurora.org>
Link: https://lore.kernel.org/r/b6d3c4dce571d78a532fd74f27def0d5dc8d8a24.1607286887.git.lukas@wunner.de
Signed-off-by: Mark Brown <broonie@kernel.org>