[sparse] define functions static, use NULL as NULL pointer
fixes:
arch/sandbox/board/hostfile.c:32:9: warning: symbol 'hf_read' was not declared. Should it be static?
arch/sandbox/board/hostfile.c:43:9: warning: symbol 'hf_write' was not declared. Should it be static?
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>