net: ipa: don't assume 8 modem routing table entries

Currently all platforms are assumed allot 8 routing table entries
for use by the modem.  Instead, add a new configuration data entry
that defines the number of modem routing table entries, and record
that in the IPA structure.

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