The locking rules in the driver came from era when sysfs attributes
could live past the point of time when device would be unbound from
the driver, and so used module-global semaphore (potentially shared
between multiple yealink devices). Thankfully these times are long
gone and attributes will not be accessible once they are removed.

Simplify the logic by moving to per-device mutex, stop checking if
there is driver data instance attached to the interface, and use
guard notation to acquire the mutex.
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20240710234855.311366-2-dmitry.torokhov@gmail.comSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>