net: ipa: kill ipa_modem_init()

A recent commit made disabling the SMP2P "setup ready" interrupt
unrelated to ipa_modem_stop().  Given that, it seems fitting to get
rid of ipa_modem_init() and ipa_modem_exit() (which are trivial
wrapper functions), and call ipa_smp2p_init() and ipa_smp2p_exit()
directly instead.

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