cpucontrol: check for the supposed firmware file type and skip
non-regular entry.

do_update() skips entries with DT_DIR type. This does not eliminate
other entries that might exist in the directory. More, since NFS
might fill d_type with DT_UNKNOWN, dot and dotdot entries are not
skipped, then mmap(2) call failed for them when update microcode files
are located on NFS.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week