drivers: staging: iio: meter: Removed unnecessary variable

Variable ret is used only to store the return value. Hence ret is
removed and the return statement modified. Coccinelle was used to
detect such removable variables:

@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>