drivers: staging: iio: meter: Removed unnecessary variable

Variable ret is used only to store the error code to be returned.
Hence use of ret is removed and the return statement modified.
Coccinelle was used to prepare the patch:

@rule1@
identifier ret;
expression e;
@@

-ret =
+return
        e;
-return ret;

Signed-off-by: Tina Johnson <tinajohnson.1234@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>