drivers/usb/ehci.h: Use C99 flexible arrays

Use C99 flexible arrays instead of older style of one-element or
zero-length arrays.
It allows the compiler to generate errors when the flexible array does
not occur at the end in the structure.

Change-Id: Ideed4b333632df5068b88dde6f89d3831e3046d1
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/76840
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Maximilian Brune <maximilian.brune@9elements.com>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>

src/drivers/usb/ehci.h

@@ -97,7 +97,7 @@ struct ehci_regs {
 #define FLAG_CF		(1<<0)		/* true: we'll support "high speed" */
 
 	/* PORTSC: offset 0x44 */
-	u32		port_status[0];	/* up to N_PORTS */
+	u32		port_status[];	/* up to N_PORTS */
 /* EHCI 1.1 addendum */
 #define PORTSC_SUSPEND_STS_ACK 0
 #define PORTSC_SUSPEND_STS_NYET 1