drivers: spi_nrfx_spis: Refactor prepare_for_transfer()

Refactor the function to make the execution flow in transceive()
clearer. In particular, return error codes directly, not through
spi_context_complete() which is unnecessary in this case.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>