staging: iio: meter: remove unused variable

staging: iio: meter: remove unused variable

This patch removes a variable that was simply used to
store the return value of a function call before
returning it.

The issue was detected and resolved using the following
coccinelle script:

@@
identifier len,f;
@@

-int len;
 ... when != len
     when strict
-len =
+return
        f(...);
-return len;

Signed-off-by: Aya Mahfouz <mahfouz.saif.elyazal@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>