[media] ts2020: Add a comment about lifetime of on-stack pdata in ts2020_attach()

ts2020_attach() allocates a variable pdata on the stack and then passes a
pointer to it to i2c_new_device() which stashes the pointer in persistent
structures.

Add a comment to the effect that this isn't actually an error because the
contents of the variable are only used in ts2020_probe() and this is only
called ts2020_attach()'s stack frame exists.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>