soc/amd/picasso: mark usb2_phy_tune struct as packed

Since the binary layout of this struct matters, it should be marked as
packed. Since all struct elements are uint8_t, this shouldn't result in
a different layout though.

BUG=b:161923068

Change-Id: I6a390c3a3f35eaf8a72928b4cef0e9f405770619
Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/43780
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/amd/picasso/chip.h

@@ -16,7 +16,7 @@
 /*
   USB 2.0 PHY Parameters
 */
-struct usb2_phy_tune {
+struct __packed usb2_phy_tune {
 	/* Disconnect Threshold Adjustment. Range 0 - 0x7 */
 	uint8_t	com_pds_tune;
 	/* Squelch Threshold Adjustment. Range 0 - 0x7 */