efi.h declares a function type, and then uses that as an argument to
another function, and expects the compiler to magically demote the
function to a function pointer.

Even a gcc person (rth) was surprised that this was legal, and it
doesn't match any other use of a function pointer in the kernel, and
sparse doesn't like the implicit type-conversion.

So make the type sane in the first place, instead of depending on
a very weird corner case of the C language.