tty: rfcomm: prefer array indexing over pointer arithmetic

Refactor the list_for_each_entry() loop of rfcomm_get_dev_list()
function to use array indexing instead of pointer arithmetic.

This way, the code is more readable and idiomatic.

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Erick Archer <erick.archer@outlook.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>