Staging: pi433: declare functions static

Declare functions static to fix sparse warnings:

warning: symbol 'pi433_receive' was not declared. Should it be static?
warning: symbol 'pi433_tx_thread' was not declared. Should it be static?

Signed-off-by: Joseph Wright <rjosephwright@gmail.com>
Reviewed-by: Marcus Wolf <linux@wolf-entwicklungen.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>