rtl8xxxu: avoid parsing short RX packet

One USB data buffer can contain multiple received network
packets. If that's the case, they're processed this way:
1. Original buffer is cloned
2. Original buffer is trimmed to contain only the first
   network packet
3. This first network packet is passed to network stack
4. Cloned buffer is trimmed to eliminate the first network
   packet
5. Repeat with the cloned buffer until there are no more
   network packets inside

However, if the space remaining in original buffer after
the first network packet is not enough to contain at least
another network packet descriptor, it is not cloned.

The loop parsing this packets ended if remaining space == 0.
But if the remaining space was > 0 but < packet descriptor
size, another iteration of the loop was done, processing again
the previous packet because cloning didn't happen. Moreover,
the ownership of this packet had been passed to network
stack in the previous iteration.

This patch ensures that no extra iteration is done if the
remaining size is not enough for one packet, and also avoid
the first iteration for the same reason.

Probably this doesn't happen in practice, but can happen
theoretically.

Signed-off-by: Íñigo Huguet <ihuguet@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210511071926.8951-1-ihuguet@redhat.com