net: ipa: rename two transaction fields

There are two fields in a GSI transaction that keep track of TRE
counts.  The first represents the number of TREs reserved for the
transaction in the TRE ring; that's currently named "tre_count".
The second is the number of TREs that are actually *used* by the
transaction at the time it is committed.

Rename the "tre_count" field to be "rsvd_count", to make its meaning
a little more specific.  The "_count" is present in the name mainly
to avoid interpreting it as a reserved (not-to-be-used) field.  This
name also distinguishes it from the "tre_count" field associated
with a channel.

Rename the "used" field to be "used_count", to match the convention
used for reserved TREs.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>