staging: rtl8712: Fix possible buffer overflow in r8712_sitesurvey_cmd

Function r8712_sitesurvey_cmd calls memcpy without checking the length.
A user could control that length and trigger a buffer overflow.
Fix by checking the length is within the maximum allowed size.

Signed-off-by: Lee Gibson <leegib@gmail.com>
Link: https://lore.kernel.org/r/20210301132648.420296-1-leegib@gmail.com


Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>