Changes

ipa: Split msgb generation into its own helper function

This allows having a clearer picture when comparing against other
protocol stacks like M3UA and SUA, since both have a sua_to_msg() and
m3ua_to_msg() functions (which in turn call xua_to_msg()).

This way ipa_tx_xua_as() also becomes much more similar to
m3ua_tx_xua_as() and sua_tx_xua_as().

Change-Id: Ic0a405ab4d1811efea137167dcb08c9308a4d7e7