struct has_ddk_get_protocol

Mixin checks: ensure that a type meets the following qualifications:

1) has a method with the correct name (this makes the compiler errors a little more sane),

2) inherits from ddk::Device (by checking that it inherits from ddk::internal::base_device), and

3) has the correct method signature.

Note that the 3rd requirement supersedes the first, but the static_assert doesn't even compile if

the method can't be found, leading to a slightly more confusing error message. Adding the first

check gives a chance to show the user a more intelligible error message.