qcacld-3.0: Exclude length field size from max beacon payload size

Don't account length field size while comparing against max
beacon size.

Current driver starts from (ptr+4 bytes) then tries to copy
512 bytes from that point which creates to copy extra 4 bytes
beyond the array's boundary.

Instead copy only 508 bytes if driver starts copying from
(ptr+ 4bytes).

 ptr
  ^
  |
  |
  +---------------+--------------------------------+
  |               |                                |
  |  Length       |   Max Beacon payload           |
  |               |                                |
  +---------------+--------------------------------+

  |<-- 4 bytes -->|<-------- 508 bytes ----------->|

  |<-------------  512 bytes --------------------->|

CRs-Fixed: 2327052
Change-Id: I2646986ec424f7da31107ad01f673588734eaa52
1 file changed