net: ipa: introduce ipa_cmd_init()

Currently, ipa_cmd_data_valid() is called by ipa_mem_config().
Nothing it does requires access to hardware though, so it can be
done during the init phase of IPA driver startup.

Create a new function ipa_cmd_init(), whose purpose is to do early
initialization related to IPA immediate commands.  It will call the
build-time validation function, then will make the two calls made
previously by ipa_cmd_data_valid().  This make ipa_cmd_data_valid()
unnecessary, so get rid of it.

Rename ipa_cmd_header_valid() to be ipa_cmd_header_init_local_valid(),
so its name is clearer about which IPA immediate command it is
associated with.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>