[x86] remove uses of magic macros for boot_params access

Instead of using magic macros for boot_params access, simply use the
boot_params structure.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
diff --git a/arch/x86/mach-visws/setup.c b/arch/x86/mach-visws/setup.c
index 1f81f10..de4c9db 100644
--- a/arch/x86/mach-visws/setup.c
+++ b/arch/x86/mach-visws/setup.c
@@ -152,7 +152,7 @@
 {
 	long long gfx_mem_size = 8 * MB;
 
-	mem_size = ALT_MEM_K;
+	mem_size = boot_params.alt_mem_k;
 
 	if (!mem_size) {
 		printk(KERN_WARNING "Bootloader didn't set memory size, upgrade it !\n");