spi: lantiq: fix: Rx overflow error in full duplex mode

In full duplex mode, rx overflow error is observed. To overcome the error,
wait until the complete data got received and proceed further.

Fixes: 17f84b79 ("spi: lantiq-ssc: add support for Lantiq SSC SPI controller")
Signed-off-by: Dilip Kota <eswara.kota@linux.intel.com>
Link: https://lore.kernel.org/r/efb650b0faa49a00788c4e0ca8ef7196bdba851d.1594957019.git.eswara.kota@linux.intel.com


Signed-off-by: Mark Brown <broonie@kernel.org>