gpu: nova-core: falcon: align DMA transfers to 256 bytes

gpu: nova-core: falcon: align DMA transfers to 256 bytes

Falcon DMA transfers are done in 256 bytes increments, and the method
responsible for initiating the transfer checked that the required length
was indeed a multiple of 256. While correct, this also requires callers
to specifically account for this limitation of DMA transfers, and we had
for instance the fwsec code performing a seemingly arbitrary (and
potentially overflowing) upwards alignment of the DMEM load size to
match this requirement.

Let's move that alignment into the loading code itself instead: since it
is working in terms of number of transfers, we can turn this upwards
alignment into a non-overflowing operation, and check that the requested
transfer remains into the limits of the DMA object. This also allows us
to remove a DMA-specific constant in the fwsec code.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://lore.kernel.org/r/20250821-falcondma_256b-v2-1-83e8647a24b5@nvidia.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>