net: ipa: add support for inline checksum offload

Starting with IPA v4.5, the IPA hardware can implement checksum
offload using QMAP v5 protocol.  Previously, only QMAP v4 offload
was supported.

For both QMAP v4 and v5, the RMNet driver is involved in preparing
TX packets for offload, or for interpreting whether hardware verifies
a packet's checksum is good.  But the IPA driver must program the
hardware to implement the desired offload type for an endpoint.

Change the checksum offload field for endpoint data to be an
enumerated type rather than a Boolean.  Its three values represent
disabled (0), enabled (QMAP v4), or inline (QMAP v5) offlad.  Update
ipa_endpoint_init_cfg() to program the proper checksum offload type.

Create a helper function to compute the header size expected for an
endpoint, depending on the endpoint's checksum offload type.

Signed-off-by: Alex Elder <elder@linaro.org>