hw/misc/aspeed_hace: Extract accumulation-mode hash execution into helper function

hw/misc/aspeed_hace: Extract accumulation-mode hash execution into helper function

To improve code readability and maintainability of do_hash_operation(), this
commit introduces a new helper function: hash_execute_acc_mode().

This function encapsulates the full flow for accumulation mode, including
context initialization, update, conditional finalization, and digest writeback
with I/O vector unmapping.

No functional changes are introduced.

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250515081008.583578-9-jamin_lin@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>