fields.rs: Make request_type function a const fn

[ammubhave]

Make request_type function in fields.rs a const fn so that it can used
to create const global variables.

Test: The following code now compiles

const CTRL_OUT: u8 = request_type(
    rusb::Direction::Out,
    rusb::RequestType::Vendor,
    rusb::Recipient::Device,
);

[a1ien]

LGTM. Thanks