[ns] adding a function to change the value of CWmax_ for the backoff procedure

[sasan zhalehpour]

hi,
i want to add a function in mac802_11.{cc,h} to change the value of CWMax_
in TCL code
i mean when ever i invoke it during the execution of TCL code, this function
could change the value of CWmax_

Any help will be highly appreciated.
regards,
sasan