spi: qup: refactor spi_qup_prep_sg

Take specific sgl and nent to be prepared.  This is in
preparation for splitting DMA into multiple transacations, this
contains no code changes just refactoring.

Signed-off-by: Matthew McClintock <mmcclint@codeaurora.org>
Signed-off-by: Varadarajan Narayanan <varada@codeaurora.org>
Signed-off-by: Mark Brown <broonie@kernel.org>