usb: ftdi-elan: Convert "mutex" to semaphore

The "mutex" ftdi->sw_lock is used as a lock and a completion. Convert
it to a real semaphore which allows both.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>