spi: spi-davinci: Fix variable type

'prescale' contains the return value of davinci_spi_get_prescale()
which is a signed integer. Convert 'prescale' to integer to silence
the following warning:
drivers/spi/spi-davinci.c:318 davinci_spi_setup_transfer() warn:
unsigned 'prescale' is never less than zero.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>