net: ipa: eliminate legacy arguments

We enable a channel doorbell engine only for IPA v3.5.1, and that is
now handled directly by gsi_channel_program().

When initially setting up a channel, we want that doorbell engine
enabled, and we can request that independent of the IPA version.

Doing that makes the "legacy" argument to gsi_channel_setup_one()
unnecessary.  And with that gone we can get rid of the "legacy"
argument to gsi_channel_setup(), and gsi_setup() as well.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>