hw/misc/aspeed_hace: Improve readability and consistency in variable naming

hw/misc/aspeed_hace: Improve readability and consistency in variable naming

Currently, users define multiple local variables within different if-statements.
To improve readability and maintain consistency in variable naming, rename the
variables accordingly.
Introduced "sg_addr" to clearly indicate the scatter-gather mode buffer address.

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250515081008.583578-3-jamin_lin@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>