spi/qspi: Fix qspi remove path.

There is a bug in qspi removal path, as a result of which
qspi cannot be removed when used as a module. The patch
solves the bug and qspi can be removed cleanly.

The bugs fixed are:
-pm_runtime used around register access.
- pm_runtime_disable need to be done before removal.
- spi_unregister_master need to be called to unregister
   the spi device.
Tested on DRA7 board.

Signed-off-by: Sourav Poddar <sourav.poddar@ti.com>
Signed-off-by: Mark Brown <broonie@linaro.org>