spi: dw: Disable clock after unregistering the host

The dw_mmio driver disables the block clock before unregistering
the host. The code unregistering the host may access the SPI block
registers. If register access happens with block clock disabled,
this may lead to a bus hang. Disable the clock after unregistering
the host to prevent such situation.

This bug was observed on Altera Cyclone V SoC.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>