The config_rom struct members are only accessed during relatively
infrequent self-ID-complete interrupts and only if the local config ROM
was changed, while the ar_, at_, ir_, it_ members are used very
frequently during I/O.  Hence move the config_rom members further down.

More importantly, make the huge self_id_buffer member the last one; this
is only accessed in self-ID-complete interrupts.
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>