spi: spi_qsd: Allow manual binding/unbinding of driver.

The spi_qsd driver is prevented from manually being unbound from sysfs.
Change the init call to platform_driver_register and remove the probe
function and subroutines it calls from the init section.

Change-Id: I5a0f66a38d6844dc9283569deb703bf2e8a490a8
CRs-Fixed: 734063
Signed-off-by: Dan Sneddon <dsneddon@codeaurora.org>