gpu: nova-core: Simplify `transmute` and `transmute_mut` in fwsec.rs

gpu: nova-core: Simplify `transmute` and `transmute_mut` in fwsec.rs

This patch solves one of the existing mentions of COHA, a task
in the Nova task list about improving the `CoherentAllocation` API.
It uses the new `from_bytes` method from the `FromBytes` trait as
well as the `as_slice` and `as_slice_mut` methods from
`CoherentAllocation`.

Signed-off-by: Daniel del Castillo <delcastillodelarosadaniel@gmail.com>
[acourbot@nvidia.com: set prefix to "gpu: nova-core:".]
[acourbot@nvidia.com: fix merge conflict after imports refactor.]
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Message-ID: <20251104193756.57726-1-delcastillodelarosadaniel@gmail.com>