spi: zynqmp_gqspi: Switch genfifo start to manual mode

Current implementation uses auto mode for starting generic FIFO.
The recommendation from IP designers is to use manual mode, hence
change to manual start mode.

In fill genfifo first write to genfio and then trigger manual start.

Also enable and check for genfifo empty interrupt status in place of
genfifo not full interrupt.

Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@xilinx.com>
Tested-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>