e1000: Rename 'struct adapter' to 'struct e1000_sc'

Rename the 'struct adapter' to 'struct e1000_sc' to avoid type ambiguity
in things like kgdb.

Reviewed by: jhb, markj
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D32129

(cherry picked from commit dc9260515449cde9a4b26b5448f7386388c55bbd)