teletext: fix extractPTS

copy extract_pts implementation from subtitle.cpp
reducing the number of bitshifts, and fixing the
LSB offset
(see 3b24f0d476f16733823852e3778360a8276d9b59)

NOTE: the 7 least significant PTS bits account
for 1.41ms, so the LSB offset bug would not have
had a noticable effect on teletext subtitle timing