xpp: FXO: in-firmware polarity-reversal detection

PIC_TYPE_2 rev.11039 adds support for polarity reversal detection. Support
those messages.

Signed-off-by: Tzafrir Cohen <tzafrir.cohen@xorcom.com>
xpp: Serialize dahdi registration

xpp: Serialize dahdi registration: Cause races under highly-parallel
workloads (with dahdi_autoreg=0).

Signed-off-by: Tzafrir Cohen <tzafrir.cohen@xorcom.com>