net: ipa: count the number of modem TX endpoints

In ipa_endpoint_modem_exception_reset_all(), a high estimate was
made of the number of endpoints that need their status register
updated.  We only used what was needed, so the high estimate didn't
matter much.

However the next few patches are going to limit the number of
commands in a single transaction, and the overestimate would exceed
that.  So count the number of modem TX endpoints at initialization
time, and use it in ipa_endpoint_modem_exception_reset_all().

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