fs: semihosting: Use correct variable for error check

After calling a function that can return an error, the test to detect
that error should use the return value not a different variable. Fix it.

This issue was found by Smatch.

Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>
Reviewed-by: Sean Anderson <sean.anderson@seco.com>
Fixes: f676b45151 ("fs: Add semihosting filesystem")

fs/semihostingfs.c

@@ -35,7 +35,7 @@ static int smh_fs_read_at(const char *filename, loff_t pos, void *buffer,
 	}
 	if (!maxsize) {
 		size = smh_flen(fd);
-		if (ret < 0) {
+		if (size < 0) {
 			smh_close(fd);
 			return size;
 		}