net: ipa: determine the maximum endpoint ID

Each endpoint ID has an entry in the IPA endpoint array.  But the
size of that array is defined at compile time.  Instead, rename
ipa_endpoint_data_valid() to be ipa_endpoint_max() and have it
return the maximum endpoint ID defined in configuration data.
That function will still validate configuration data.

Zero is returned on error; it's a valid endpoint ID, but we need
more than one, so it can't be the maximum.  The next patch makes use
of the returned maximum value.

Finally, rename the "initialized" mask of endpoints defined by
configuration data to be "defined".

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>