mac80211.c: Remove or mark unused function parameters

Compiling with "-W -Wall -Wextra" reveals the following warnings in
mac80211.c:

mac80211.c: In function ‘nl80211_init’:
mac80211.c:78:67: warning: unused parameter ‘device’ [-Wunused-parameter]
mac80211.c: In function ‘nl80211_wait_handler’:
mac80211.c:106:48: warning: unused parameter ‘msg’ [-Wunused-parameter]
mac80211.c: In function ‘nl80211_error_handler’:
mac80211.c:115:54: warning: unused parameter ‘nla’ [-Wunused-parameter]
mac80211.c:117:12: warning: unused parameter ‘arg’ [-Wunused-parameter]
mac80211.c: In function ‘nl80211_del_mon_if’:
mac80211.c:181:72: warning: unused parameter ‘device’ [-Wunused-parameter]

Fix them by either marking them as unused (where we need to conform to
library APIs or remove them alltogether (for our own APIs). For the
function leave_rfmon_mac80211() the according users (netsniff-ng and
trafgen) are also changed.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>