spi: cadence-quadspi: Handle spi_unregister_master() in remove()

Currently devres managed removal of the spi_controller happens after
removing the power domain of the host platform_device.While this
does not affect the clean removal of the controller, but affects
graceful removal of the child devices if the child  device removal
requires issuing commands over SPI.

Eg. flash device being soft reset to 1S-1S-1S mode before removal
so that on next probe operations in 1S-1S-1S mode is successful.

Failure is seen when `rmmod spi-cadence-quadspi` is performed:

root@j7-evm:~# rmmod spi_cadence_quadspi
[ 49.230996] cadence-qspi 47050000.spi: QSPI is still busy after 500ms timeout.
[ 49.238209] spi-nor spi1.0: operation failed with -110
[ 49.244457] spi-nor spi1.0: Software reset failed: -110

and on subsequent modprobe the OSPI flash probe fails as it
is in 8D-8D-8D mode since the previous soft reset did not happen.

root@j7-evm:~# modprobe spi_cadence_quadspi
[ 73.253536] spi-nor spi0.0: unrecognized JEDEC id bytes: ff ff ff ff ff ff
[ 73.260476] spi-nor: probe of spi0.0 failed with error -2

This commit adds necessary changes to perform spi_unregister_master()
in the host device remove() so that the child devices are gracefully
removed before the power domain is removed.

changes tested on J721E with mt35xu512aba flash.

Signed-off-by: Vaishnav Achath <vaishnav.a@ti.com>
Link: https://lore.kernel.org/r/20220511115516.14894-1-vaishnav.a@ti.com
Signed-off-by: Mark Brown <broonie@kernel.org>