spi: spi-cadence: Move TX FIFO full busy-wait into FIFO

spi: spi-cadence: Move TX FIFO full busy-wait into FIFO

SPI host transfers could intermittently stall with spi_transfer timeouts.
The TXFULL condition was checked only once in cdns_transfer_one() before
cdns_spi_process_fifo(), so if the FIFO became full again during refill,
writes could be dropped and the transfer would never complete.

Move the TXFULL busy-wait into the TX path of cdns_spi_process_fifo() so
the 10µs back-off is applied per FIFO entry during filling, ensuring
forward progress and eliminating spurious timeouts.

Restrict the delay to host mode using spi_controller_is_target(), the
controller is passed into cdns_spi_process_fifo() so the check is made at
the point of use. In target mode this delay must not run as it causes the
target to miss its transfer window and corrupt data.

Fixes: 49530e641178 ("spi: cadence: Add usleep_range() for cdns_spi_fill_tx_fifo()")
Signed-off-by: Srikanth Boyapally <srikanth.boyapally@amd.com>
Reviewed-by: Radhey Shyam Pandey <radhey.shyam.pandey@amd.com>
Link: https://patch.msgid.link/20260720125510.60166-1-srikanth.boyapally@amd.com
Signed-off-by: Mark Brown <broonie@kernel.org>