ibmvnic: rename local variable index to bufidx

The local variable 'index' is heavily used in some functions and is
confusing with the presence of other "index" fields like pool->index,
->consumer_index, etc. Rename it to bufidx to better reflect that its
the index of a buffer in the pool.

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>