synclink: kill the big kernel lock

We don't need it while waiting and we can lock the ioctls using the port
mutex. While at it eliminate use of the hangup mutex and switch to the port
mutex.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>