net: ipa: use struct_size() for the interconnect array

In review for commit 8ee7ec48

 ("net: ipa: embed interconnect
array in the power structure"), Jakub Kicinski suggested that a
follow-up patch use struct_size() when computing the size of the
IPA power structure, which ends with a flexible array member.

Do that.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Alex Elder <elder@linaro.org>
Link: https://lore.kernel.org/r/20220311162423.872645-1-elder@linaro.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>