net: ipa: be explicit about endianness

Sparse warns that the assignment of the metadata mask for a QMAP
endpoint in ipa_endpoint_init_hdr_metadata_mask() is a bad
assignment.  We know we want the mask value to be big endian, even
though the value we write is in host byte order.  Use a __force
tag to indicate we really mean it.

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