spi: davinci: simplify prescalar calculation

Simplify pre-scalar calculation and move it into a seprate
function.

Refuse to correct invalid pre-scalar values silently as this
might lead to unexpected bugs and lower performance. Instead
an error will force users to dig into the root-cause of the
issue.

While at it, remove some device specific checks on the maximum
SPI frequency. As the driver supports the SPI interface
implemented on various devices, it should only take care of core
SPI limitations and leave the device specific handling to platform
code.

Signed-off-by: Brian Niebuhr <bniebuhr@efjohnson.com>
Tested-By: Michael Williamson <michael.williamson@criticallink.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>