Transceiver52M: Reset overrun and underrun indicators

Underruns are only explicitly set on the downlink side. Overruns
are logged but unused. In either case, reset indicators to false
to avoid sending false state information.

Signed-off-by: Thomas Tsou <tom@tsou.cc>

git-svn-id: http://wush.net/svn/range/software/public/openbts/trunk@6817 19bc5d8c-e614-43d4-8b26-e1612bc8e597