net: prestera: validate firmware header length

net: prestera: validate firmware header length

prestera_fw_hdr_parse() reads the firmware header before checking
that the firmware image contains that header.

Reject images shorter than struct prestera_fw_header before decoding the
magic and version fields.

Fixes: 4c2703dfd7fabb ("net: marvell: prestera: Add PCI interface support")
Signed-off-by: Pengpeng Hou <pengpeng@iscas.ac.cn>
Acked-by: Elad Nachman <enachman@marvell.com>
Link: https://patch.msgid.link/20260731141500.1-prestera-v2-pengpeng@iscas.ac.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>