net: ipa: kill two constant symbols

The entries in each IPA routing table are divided between the modem
and the AP.  The modem always gets some number of entries located at
the base of the table; the AP gets all those that follow.

There's no reason to think the modem will use anything different
from the first entries in a routing table, so:
  - Get rid of IPA_ROUTE_MODEM_MIN (just assume it's 0)
  - Get rid of IPA_ROUTE_AP_MIN (just assume it's IPA_ROUTE_MODEM_COUNT)
And finally:
  - Open-code IPA_ROUTE_AP_COUNT and remove its definition

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>