staging: greybus: uart: Fix atomicity violation in get_serial_info()

staging: greybus: uart: 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
meaningless.

Signed-off-by: Qiu-ji Chen <chenqiuji666@gmail.com>
Fixes: 0aad5ad563c8 ("greybus/uart: switch to ->[sg]et_serial()")
Reviewed-by: Johan Hovold <johan+linaro@kernel.org>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Alex Elder <elder@riscstar.com>
Link: https://lore.kernel.org/r/20241107113337.402042-1-chenqiuji666@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>