net: ipa: fix two minor ipa_cmd problems

In "ipa_cmd.h", ipa_cmd_data_valid() is declared, but that function
does not exist.  So delete that declaration.

Also, for some reason ipa_cmd_init() never gets called.  It isn't
really critical--it just validates that some memory offsets and a
size can be represented in some register fields, and they won't fail
with current data.  Regardless, call the function in ipa_probe().

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