net: ipa: define per-endpoint receive buffer size

Allow RX endpoints to have differing receive buffer sizes.  Define
the receive buffer size in the configuration data, and use that
rather than IPA_RX_BUFFER_SIZE when configuring the endpoint.

Add verification in ipa_endpoint_data_valid_one() that the receive
buffer specified for AP RX endpoints is both big enough to handle at
least one full packet, and not so big in an aggregating endpoint
that its size can't be represented when programming the hardware.
Move aggr_byte_limit_max() up in "ipa_endpoint.c" so it can be used
earlier in the file without a forward-reference.

Initially we'll just keep the 8KB receive buffer size already in use
for all AP RX endpoints..

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