net: qualcomm: rmnet: Improve header-only packet check

After copying the headers into a newly allocated SKB during multi-NLO
RSB/RSC processing, the original headers needed to be pulled from the
primary descriptor to avoid adding them twice when populating the data.
As part of this, a check was made to accelerate packets that contained
only headers and no data payload though the processing. This check was
mistakenly performed for the secondary descriptors as well, which do not
have the L3 and L4 header length included in their internal length. This
caused secondary descriptors which had payloads exactly as long as the
combined header length to be mistakenly accelerated without appending the
packet data. This length check is now performed only for the primary
descriptor.

Change-Id: I3fca6acea378d470e2fd4ebee363c13931edaa51
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>