V4L/DVB (7183): radio-si470x: fix build warning

fix the following build warning:

radio-si470x.c: In function 'si470x_get_rds_registers':
radio-si470x.c:562: warning: format '%ld' expects type 'long int',
		    but argument 3 has type 'unsigned int'

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>