hw/misc/aspeed_hace: Extract digest write and iov unmap into helper function

hw/misc/aspeed_hace: Extract digest write and iov unmap into helper function

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

The helper consolidates the final digest writeback and subsequent unmapping of
the I/O vectors into a single routine.

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/20250515081008.583578-7-jamin_lin@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>