apei: Fix i386 build over bus read, write function

bus_{read,write}_8 are macro wrappers around the corresponding bus_space
functions in sys/bus.h, so implementing bus_{read,write}_8 won't work.
Implement the underlying bus_space function instead.

Reviewed by: jrtc27, rlibby
Fixes: 9313f6b01485
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D58301