USB: auerswald: Convert stats_sem in a mutex

The semaphore cp->mutex is used as mutex, convert it to the mutex API

Signed-off-by: Matthias Kaehlcke <matthias@kaehlcke.net>
Cc: Wolfgang Mües <wolfgang@iksw-muees.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>