If the calls to device_reset() or devm_spi_register_controller() fail on
probe of the MediaTek MT7621 SPI driver, the spi_controller struct is
erroneously not freed.  Fix by switching over to the new
devm_spi_alloc_master() helper.

Additionally, there's an ordering issue in mt7621_spi_remove() wherein
the spi_controller is unregistered after disabling the SYS clock.
The correct order is to call spi_unregister_controller() *before* this
teardown step because bus accesses may still be ongoing until that
function returns.

All of these bugs have existed since the driver was first introduced,
so it seems fair to fix them together in a single commit.

Fixes: 1ab7f2a4 ("staging: mt7621-spi: add mt7621 support")
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Stefan Roese <sr@denx.de>
Cc: <stable@vger.kernel.org> # v4.17+: 5e844cc3: spi: Introduce device-managed SPI controller allocation
Cc: <stable@vger.kernel.org> # v4.17+
Link: https://lore.kernel.org/r/72b680796149f5fcda0b3f530ffb7ee73b04f224.1607286887.git.lukas@wunner.deSigned-off-by: Mark Brown <broonie@kernel.org>