wifi: rtw89: fw: validate multi-firmware header before accessing

A firmeware file contains multi-firmware with a header to represent
contents. The mfw_hdr->fw_nr is to define number of firmware in file.

         +-----+-------+------+---------+--------------+
         | sig | fw_nr | rsvd | version | reserved     |
         +---------------------------------------------+ --
 fw 0    | cv | type | mp | rsvd | shift | size | rsvd |   \
         +---------------------------------------------+   |
 fw 1    | cv | type | mp | rsvd | shift | size | rsvd |   | mfw_hdr->fw_nr
         +---------------------------------------------+   |
 fw N-1  |                  ...                        |   /
         +=============================================+ --
         |               fw 0 content                  |
         |       (pointed by fw0 shift/size)           |
         +=============================================+

To avoid Coverity warning, validate header is in range of firmware size,
and also validate the range of actual firmware content is in range.

Addresses-Coverity-ID: 1494046 ("Untrusted loop bound")

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20250203072911.47313-4-pkshih@realtek.com