wctdm24xxp: Only two polarity reversals are needed to validate RING on FXO ports.

This fixes a regression introduced in commit r10186 "wctdm24xxp: Use time
interval for debouncing FXO ring detect." [1] which was first released in
DAHDI-Linux 2.6.0. This only affects users with analog trunks whose providers do
not present 4 polarity reversals on the ring signals. The reporter of this issue
is based in South Africa.

[1] http://svnview.digium.com/svn/dahdi?view=revision&revision=10168

In prior versions, the ring detector did not check for polarity reversals, only
the presence of ringing voltage unless fwringdetect or neonmwi_monitor mode was
set, and even when one of those modes were set, the driver only needed two
reversals to validate a ring. This commit allows the driver to always stay in
fwringdetect mode but restores the requirement for only two reversals.

Also included in this commit is a change to ensure that ringing is not reported
when debouncing lost battery which can happen when voltage is swinging through
0.

Reported-and-Tested-by: Jaco Kroon <jaco@uls.co.za>
Internal-Issue-ID: DAHLIN-298
Signed-off-by: Shaun Ruffell <sruffell@digium.com>

git-svn-id: http://svn.asterisk.org/svn/dahdi/linux/trunk@10719 a0bf4364-ded3-4de4-8d8a-66a801d63aff
(cherry picked from commit 84e70cdac5ca14a43f37cf083f241eb7eb952bc5)