spi_qsd: set bus number based on the cell-index property

When using device tree, the bus number needs to be set based on the
cell-index property rather than the 'id' from the platform device.
This makes sure that the SPI bus number is set correctly.

Change-Id: I4489c28363a7fd79f3bc3db76207a1bc8ef507cb
Signed-off-by: Kenneth Heitke <kheitke@codeaurora.org>