drivers: spi_mcux_lpspi: Consolidate IRQ handle

Small change to consolidate the amount of lines of code in
the spi_mcux_isr by making a macro for the argument and removing
a line of code that was commented out.

Also move the ISR to be the first function in the file which is
common in many other drivers, instead of randomly in the middle of the
file. And move the isr callback to be next to the isr.

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