wifi: rtw89: fw: shrink download size of security section for RTL8852B

For RTL8852B, when current firmware is secure boot, the security section
needs a special treatment that shrink its size to 960.

As figure below, not only shrink the amount of download size of security
section (2), but also need to modify the section size in firmware header
(1) that is also downloaded to chip.

   +---------------------------+
   |      firmware header      |
   |                           |
   | +-----------------------+ |
   | | section type, size N -|-|-------+
   | | ...               (1) | |       |
   | +-----------------------+ |       |
   +---------------------------+       | 2048 shrink to 960
   :                           :       |
   +---------------------------+ -\    |
   |  security section type 9  |  |    |
   |           (2)             |  | <--+
   |                           |  |
   +---------------------------+ -/

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20241030022135.11688-5-pkshih@realtek.com