staging: pi433: fix race condition in pi433_open

The device structure contains a useless non-atomic users counter which
is subject to race conditions. It has probably been created to handle
the case where remove is executed while operations are still executing
on open fds but this will never happen because of reference counts.

Drop the users counter and move rx buffer {de,}allocation to probe()
and remove(). Remove associated dead code from open() and release().
Remove related TODO entry from ioctl().

Signed-off-by: Hugo Lefeuvre <hle@owl.eu.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>