[media] si470x: don't use buffer on the stack for USB transfers

You shouldn't use buffers allocated on the stack for USB transfers,
always kmalloc them.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>

diff --cc drivers/media/radio/si470x/radio-si470x-usb.c
Simple merge

diff --cc drivers/media/radio/si470x/radio-si470x.h
Simple merge