Transceiver52M: Reapply "Set B2XX clock frequency to 26 MHz"

Patch e5aa3f99752e388131695cc380f7a549d914f56f "Set B2XX clock frequency
to 26 MHz" was inadvertently reverted by
c13f8bde8f1d00f44cc854fa77b4196a733d7808 "merged in commercial openbts".

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