media: hackrf: group device capabilities

Instead of putting V4L2_CAP_STREAMING and V4L2_CAP_READWRITE
everywhere, set device_caps earlier with these values.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>