qtnfmac: modify debugfs to support multiple cards

This patch modifies location of debugfs entries and their naming
conventions to support multiple wireless cards on pcie host.
Selected approach is to use separate directories for different
wireless cards in top-level qtnfmac debugfs directory.

Here is an example that clarifies the chosen naming conventions:

 $ sudo ls /sys/kernel/debug/qtnfmac/
   qtnfmac_pcie:0000:01:00.0

Signed-off-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>