staging: pi433: add comment to rx_lock mutex definition

Checkpatch reports: CHECK: struct mutex definition without comment.
Fix this by documenting what rx_mutex struct is used for in pi433
driver.

Signed-off-by: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com>
Link: https://lore.kernel.org/r/20220103232616.GA11161@mail.google.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>