spi_nxp_lpspi: Fix unit of buf_len in fill_tx_fifo

The buf_len parameter of lpspi_fill_tx_fifo is supposed to be bytes, so
we do not need to convert it. This could cause an issue if the end of
the buffer is less bytes than the word size.

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