spi: zynqmp_gqspi: Fix write issue

Enable manual start in zynqmp_qspi_fill_gen_fifo().
Also enable GQSPI_IXR_GFNFULL_MASK and check for it instead of
GQSPI_IXR_GFEMTY_MASK.

Add dummy write to genfifo register in chipselect.

Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@xilinx.com>