net: ipa: use IPA power device pointer

The ipa_power structure contains a copy of the IPA device pointer,
so there's no need to pass it to ipa_interconnect_init().  We can
also use that pointer for an error message in ipa_power_enable().

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