drivers/intel/gma/opregion.c: Get rid of device_t

Use of `device_t` has been abandoned in ramstage.

Change-Id: Idff2ec30f0b8c9a49f4369e231be92cbc8070dc0
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/26009
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/drivers/intel/gma/opregion.c

@@ -30,7 +30,7 @@
 /* Write ASLS PCI register and prepare SWSCI register. */
 void intel_gma_opregion_register(uintptr_t opregion)
 {
-	device_t igd;
+	struct device *igd;
 	u16 reg16;
 	u16 sci_reg;