media: r820t: don't crash if attach fails

As pointed by smatch:
	drivers/media/tuners/r820t.c:2374 r820t_attach() error: potential null dereference 'priv'.  (kzalloc returns null)

The current function with prints error assumes that the attach
succeeds. So, don't use it in case of failures.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>