bce: Improve tiny packets transmission performance on low frequency CPU

bce: Improve tiny packets transmission performance on low frequency CPU

Update TX HOST_IDX and HOST_BSEQ registers a little bit often; mainly to
improve timeliness of packets transmission:
The TX HOST_IDX and HOST_BSEQ registers are updated after certain amount
of TX descriptors are added to the hardware TX ring.  The default value
of the amount of TX descriptors are 8.  This value could be further
tuned by per-device sysctl node hw.bceX.tx_wreg.

The default value greatly improves tiny packets transmission performance
w/ 5709C on AMD970@800Mhz (513Kpps -> 1Mpps) and it does not increase CPU
usage on AMD970@3500Mhz (CPU usage stays @24%, 5709C could only do 1Mpps).