acpi: Fix return value in acpi_device_write_dsd_gpio()

Fix ++ as suffix and * precedence. After modification, the gpio index
can be obtained correctly.
The error was introduced in the commit making it public:
commit 01344bce

BUG=None
TEST= Can get the correct index test on nissa.

Change-Id: I7a3eb89633aaebebc8bd98ac6126c578fda23839
Signed-off-by: Jianeng Ceng <cengjianeng@huaqin.corp-partner.google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/82088
Reviewed-by: Eric Lai <ericllai@google.com>
Reviewed-by: Dolan Liu <liuyong5@huaqin.corp-partner.google.com>
Reviewed-by: Kapil Porwal <kapilporwal@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/acpi/device.c

@@ -51,7 +51,7 @@ int acpi_device_write_dsd_gpio(struct acpi_gpio *gpio, int *curr_index)
 		return ret;
 
 	acpi_device_write_gpio(gpio);
-	ret = *curr_index++;
+	ret = (*curr_index)++;
 
 	return ret;
 }