transceiver: b100: raise minimum transmit latency value
transceiver: b100: raise minimum transmit latency value

Put a floor on the transmit latency of the B100 in order to suppress
underruns in typical conditions. Empirical data from a handful of
relatively recent machines shows that the B100 will underrun when
the transmit threshold is reduced to a time of 6 and a half frames,
so we set a minimum 7 frame threshold.

The overall benefit should be marginal and may increase the
possibility of bursts arriving stale (after the trasmit deadline),
but will reduce the number of alarming UHD related messages that
appear in the log file.

This patch is UHD and B100 specific - USRP1 is unaffected.

Signed-off-by: Thomas Tsou <ttsou@vt.edu>

git-svn-id: 504cad4147f1e500be0a169b9f46375eaf875953@3980 19bc5d8c-e614-43d4-8b26-e1612bc8e597

ttsou/openbts-p2.8,ttsou/openbts-p2.8,0x7678/openbts-multi-arfcn,kheimerl/vbts-openbts,fairwaves/openbts-2.8,kheimerl/vbts-openbts,0x7678/openbts-multi-arfcn,0x7678/openbts-2.8,ttsou/openbts-p2.8,0x7678/openbts-2.8,0x7678/openbts-2.8,0x7678/openbts-multi-arfcn,kheimerl/vbts-openbts,0x7678/openbts-multi-arfcn,fairwaves/openbts-2.8,0x7678/openbts-multi-arfcn,ttsou/openbts-p2.8,kheimerl/vbts-openbts,fairwaves/openbts-2.8,0x7678/openbts-2.8,kheimerl/vbts-openbts,fairwaves/openbts-2.8,fairwaves/openbts-2.8,ttsou/openbts-p2.8,0x7678/openbts-2.8