Staging: Pi433: Bugfix for wrong argument for sizeof() in TX thread

sizeof(array) != sizeof(pointer to array)
Fixes: "staging: pi433: reduce stack size in tx thread"

Fixes: 62f39d49 ("staging: pi433: reduce stack size in tx thread")
Signed-off-by: Marcus Wolf <linux@wolf-entwicklungen.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>