wifi: rtw89: fw: safely cast mfw_hdr pointer from firmware->data

Check size of struct mfw_hdr within firmware->size before type casting
to ensure to validly dereference fields from mfm_hdr pointer. Then,
check if signature field is equal to RTW89_MFW_SIG to assert current
is multi-firmware.

Addresses-Coverity-ID: 1494046 ("Untrusted loop bound")
Addresses-Coverity-ID: 1544385 ("Untrusted array index read")

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20250217064308.43559-6-pkshih@realtek.com