hw/misc/aspeed_hace: Extract direct mode hash buffer setup into helper function

hw/misc/aspeed_hace: Extract direct mode hash buffer setup into helper function

To improve code readability and maintainability of do_hash_operation(), this
commit introduces a new helper function: hash_prepare_direct_iov().
This function encapsulates the logic for setting up the I/O vector (iov)
in direct mode (non-scatter-gather).

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/20250515081008.583578-5-jamin_lin@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>