drivers: spi_xlnx_axi_quadspi: Reduce IRQ work

This driver could end up doing a great deal of work inside the ISR when
large SPI transfers were in use, which could cause significant IRQ
latency. For the normal, non-async SPI transfer case, use events to
signal the calling thread to complete the work rather than performing
FIFO transfers inside the ISR.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>