media: vidtv: psi: extract descriptor chaining code into a helper

The code to append a descriptor to the end of a chain is repeated
throughout the psi generator code. Extract it into its own helper
function to avoid cluttering.

Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>