iio-light: Use a signed return type for ltr501_match_samp_freq()

The return type "unsigned int" was used by the ltr501_match_samp_freq()
function despite of the aspect that it will eventually return a negative
error code.
Improve this implementation detail by deletion of the type modifier then.

This issue was detected by using the Coccinelle software.
Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Acked-by: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/light/ltr501.c

@@ -180,7 +180,7 @@ static const struct ltr501_samp_table ltr501_ps_samp_table[] = {
 			{500000, 2000000}
 };
 
-static unsigned int ltr501_match_samp_freq(const struct ltr501_samp_table *tab,
+static int ltr501_match_samp_freq(const struct ltr501_samp_table *tab,
 					   int len, int val, int val2)
 {
 	int i, freq;