gpu: nova-core: firmware: move firmware request code into a function

When all the firmware files are loaded from `Firmware::new`, it makes
sense to have the firmware request code as a closure. However, since we
eventually want each individual firmware constructor to request its own
file (and get rid of `Firmware` altogether), move this code into a
dedicated function that can be called by individual firmware types.

Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20250913-nova_firmware-v6-4-9007079548b0@nvidia.com


Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>