gpu: nova-core: vbios: use checked access in `FwSecBiosImage::header`

gpu: nova-core: vbios: use checked access in `FwSecBiosImage::header`

Use checked access in `FwSecBiosImage::header` for getting the header
version since the value is firmware derived.

Fixes: 47c4846e4319 ("gpu: nova-core: vbios: Add support for FWSEC ucode extraction")
Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Eliot Courtney <ecourtney@nvidia.com>
Link: https://patch.msgid.link/20260525-fix-vbios-v5-6-e5e455251537@nvidia.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>