net: ipa: stash modem TX and RX endpoints

Rather than repeatedly looking up the endpoints in the name map,
save the modem TX and RX endpoint pointers in the netdev private
area.

Signed-off-by: Alex Elder <elder@linaro.org>
Link: https://lore.kernel.org/r/20240130192305.250915-2-elder@linaro.org


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