drivers: spi: nxp_lpspi: Fix race condition in ISR

There was a race condition where `lpspi_end_xfer` can be called multiple
times per transfer. There was the case where a TX interrupt gets
triggered without the RX interrupt being set, and TX finishes writing
its last byte. Then, `spi_context_rx_len_left() == 0` is true and
`lpspi_end_xfer` happens, but the RX interrupt is still active. Then,
when the RX interrupt happens, `lpspi_end_xfer` will get called again.

To fix that, the architecture was adjusted to only call `lpspi_end_xfer`
once no interrupts are active any more, and the disabling of the
interrupts gets used to signal the end of the TX and RX part.

Minor adjustments were necessary to use the interrupt enable signals for
this purpose; the TX irq handler had its internal order reversed,
otherwise it wasn't guaranteed that the physical transfer is finished
when we disable the interrupt.

Also, the code where the RX interrupt gets disabled had to be moved out
of the RX irq handler, because the RX interrupt also needs to be
disabled if RX is finished but no RX interrupt is currently active.

Signed-off-by: Martin Stumpf <finomnis@gmail.com>