adc: meson-saradc: uint cannot be less than zero

adc: meson-saradc: uint cannot be less than zero

timeout is declared as a uint but then tested for being less than zero
which must always fail. Change the while loop for a pre-decrement on
timeout and test timeout for being zero.

This issue was found by Smatch.

Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20250722-meson_saradc-v1-1-1ab45d53da9d@linaro.org
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>