ztex: precheck the secondary solutions to avoid hw errors
the ztex bitstreams gives back the latest checked nonce and
its hash7 value and two possible solutions.

every 250ms the latest nonce is checked and compared with hash7
to count hw errors and adapt the MHz value. one change is to
use the solutions even if the latest nonce is not correct. the
original java ztex code also does it this way.

since the second solution is often not correct we have alot
of hw errors. now we always check the second solution before
we submit it to the cgminer main code.

the java code also ignores all hw errors 500ms after a sendHash.
we now do the same. this can possibly yield in a higher MHz rate.
but the chance is so low nobody will ever notice in practice.
ztex: if we had only errors in one round we do not count the errors

to detect a totally non working fpga we only do that if the last
round had some valid nonces.

if we would count the errors the automatic megahertz adaption
would drop and never recover.
driver-ztex: react to new work in 10ms and not 250ms
driver-ztex: only reset errors if there are some

Only check the errors and print the log message if
there actually were errros.
Merge pull request #381 from denis2342/hw_errors

several changes to avoid the display of hw errors