[media] si470x: cleanup define namespace

Some architectures already use CHIPID defines:

	drivers/media/radio/si470x/radio-si470x.h:57:0: warning: "CHIPID" redefined [enabled by default]
	drivers/media/radio/si470x/radio-si470x.h:57:0: warning: "CHIPID" redefined [enabled by default]
	drivers/media/radio/si470x/radio-si470x.h:57:0: warning: "CHIPID" redefined [enabled by default]

So, use SI_foo namespace to avoid conflicts.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>