spi: zynqmp_gqspi: Fix write issue at low frequencies

With current implementation we are seeing write issues at low frequencies
below 15Mhz. Make below changes to fix the issue.

1. Remove dummy genfifo entry in zynqmp_qspi_chipselect() which was
   incorrectly added in the past

2. Enable and poll for TX_FIFO_Empty after Tx data is filled in FIFO in
   zynqmp_qspi_fill_tx_fifo().

Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@xilinx.com>
Link: https://lore.kernel.org/r/1634652780-21755-1-git-send-email-ashok.reddy.soma@xilinx.com


Signed-off-by: Michal Simek <michal.simek@xilinx.com>