net: ipa: determine filter table size from memory region

Currently we assume that any filter table contains a fixed number
of entries.  Like routing tables, the number of entries in a filter
table is limited only by the size of the IPA-local memory region
used to hold the table.

Stop assuming that a filter table has exactly 14 entries.  Instead,
determine the number of entries in a routing table by dividing its
memory region size by the size of an entry.  (Note that the first
"entry" in a filter table contains an endpoint bitmap.)

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