spi: spi_qsd: Make bus vote proportional to core clock

Make the bus bandwidth voting proportional to the core clock that the
driver will set. This will help SPI throughput when the core clock is
voted much higher than the bus clocks.

Change-Id: I8d134a281b322484238620b35757c0d86b86c3f1
Signed-off-by: Girish Mahadevan <girishm@codeaurora.org>