USB: serial: ftdi_sio: Fix atomicity violation in get_serial_info()

Our static checker found a bug where set_serial_info() uses a mutex, but
get_serial_info() does not. Fortunately, the impact of this is relatively
minor. It doesn't cause a crash or any other serious issues. However, if a
race condition occurs between set_serial_info() and get_serial_info(),
there is a chance that the data returned by get_serial_info() will be
inconsistent.

Fixes: 3ae36bed ("fdti_sio: switch to ->[sg]et_serial()")
Signed-off-by: Qiu-ji Chen <chenqiuji666@gmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>