net: ipa: kill ipa_modem_setup()

The functions ipa_modem_setup() and ipa_modem_teardown() are trivial
wrappers that call ipa_qmi_setup() and ipa_qmi_teardown().  Just
call the QMI functions directly, and get rid of the wrappers.

Improve the documentation of what setting up QMI does.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>