spi_nxp_lpspi: Optimize TX fill for less interrupt

Optimize the TX fill algorithm to have less interrupts by filling the TX
fifo as much as possible during each interrupt handle.

Before, the algorithm was just a very simple, fill the TX fifo with as
much from only the current buffer as possible, then send it and wait for
the next interrupt. Now the algorithm is to fill the TX fifo as much as
possible, even if it means reading from multiple buffers during the
interrupt.

This has the advantage from master mode of having less interrupts. And
it is very important for slave mode because the slave mode does not
control the pacing of the transfer and so therefore should fill as much
as possible whenever possible in order not to miss a deadline.

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