drivers: spi_mcux_lpspi: Remove rtio prepare_start

Remove unneeded prepare_start function since all uses of it are paired
with the start function. Also, probably should not mess with chip select
before deciding to do the transfer. And just return on some error like
the rest of the driver does instead of assert.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>