drivers: dma: rpi_pico: Correct handling of NULL filter_param

From the API documentation, `dma_api_chan_filter`` can be given a value
of NULL for `filter_param`. Match the behaviour of most implementations,
and return true. This removes misleading error messages logged during
tests (e.g. `test_tst_dma0_m2m_loop`).

Signed-off-by: Andrew Featherstone <andrew.featherstone@gmail.com>