efi/runtime-wrapper: Move workqueue manipulation out of line

efi_queue_work() is a macro that implements the non-trivial manipulation
of the EFI runtime workqueue and completion data structure, most of
which is generic, and could be shared between all the users of the
macro. So move it out of the macro and into a new helper function.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>