aspeed: Fix maximum number of spi controller

aspeed: Fix maximum number of spi controller

Commit 6de4aa8dc544 ("hw/arm/aspeed_ast27x0: Add SoC Support for AST2700
A1") extends ast2700a1 spis_num to 3, but ASPEED_SPIS_NUM defines the
maximum number of spi controller to 2, result in ehci[0] is being
overwritten in runtime.

Signed-off-by: Troy Lee <troy_lee@aspeedtech.com>
Fixes: 6de4aa8dc544 ("hw/arm/aspeed_ast27x0: Add SoC Support for AST2700 A1")
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250317065938.1902272-1-troy_lee@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>