net: wireless: bcmdhd: Set complete_flag correctly for get_batch_scan

1. Correctly indicate the complete flag based on whether
   the cache is empty or not.
2. Made the scan_id a monotonously incrementing ctr to make
   debugging easier.

Bug: 22460013

Change-Id: I2cbc14ab1b29087b1a121b582f3311b1eef9374a
Signed-off-by: Ashwin <ashwin.bhat@broadcom.com>