spi: spi-ep93xx: remove private data 'current_msg'

The currently in-flight message can be found from the spi master.
Use that instead and remove the private data pointer.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Mark Brown <broonie@kernel.org>