spi: cadence_qspi: Off by 1 in test for timeout

spi: cadence_qspi: Off by 1 in test for timeout

In cadence_qspi_apb_exec_flash_cmd the test for a timeout uses a
post-decrement on the variable retry which will result in a value of -1
after the loop exit, or it would if the variable were signed.
To fix this make retry a signed variable and test its value for being
equal to -1.

This issue was found by Smatch.

Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>