loader/efi: Make gcc friendlier by move md_dev

Move the extern struct devsw md_dev out of the function. gcc is happier
with this arrangemnt often. However, we really should move it to a
header file, but that requires a bit of a rework of md support and
config.

Sponsored by: Netflix
Reviewed by: manu, tsoome
Differential Revision: https://reviews.freebsd.org/D44008