spi: cadence: Add usleep_range() for cdns_spi_fill_tx_fifo()

spi: cadence: Add usleep_range() for cdns_spi_fill_tx_fifo()

BugLink: http://bugs.launchpad.net/bugs/1794889
[ Upstream commit 49530e6411789c1b9ea3ebc58e520c19d1c3752f ]

In case of xspi work in busy condition, may send bytes failed.
once something wrong, spi controller did't work any more

My test found this situation appear in both of read/write process.
so when TX FIFO is full, add one byte delay before send data;

Signed-off-by: sxauwsk <sxauwsk@163.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>