spi/qspi: fix missing unlock on error in ti_qspi_start_transfer_one()

Add the missing unlock before return from function ti_qspi_start_transfer_one()
in the error handling case.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Mark Brown <broonie@linaro.org>