ibmvnic: define map_txpool_buf_to_ltb()

Define a helper to map a given txpool buffer into its corresponding long
term buffer (LTB) and offset. Currently there is just one LTB per txpool
so the mapping is trivial. When we add support for multiple LTBs per
txpool, this helper will be more useful.

Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.ibm.com>
Signed-off-by: Dany Madden <drt@linux.ibm.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>