rust/conf: collapse match pattern into if

rust/conf: collapse match pattern into if

Fixes clippy lint for collapsible_match.

error: this `match` can be collapsed into the outer `if let`
  --> src/conf.rs:85:9
   |
85 | /         match val {
86 | |             "1" | "yes" | "true" | "on" => {
87 | |                 return true;
88 | |             },
89 | |             _ => {},
90 | |         }
   | |_________^
   |
help: the outer pattern can be modified to include the inner pattern
  --> src/conf.rs:84:17
   |
84 |     if let Some(val) = conf_get(key) {
   |                 ^^^ replace this binding
85 |         match val {
86 |             "1" | "yes" | "true" | "on" => {
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^ with this pattern
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#collapsible_match