drivers/usb: Replace return value -10 with variable

Coverity detects that value assigned to variable "ret" is overwritten
before it is used. Fix the issue by returning right value.

Found-by: Coverity CID 1255942, 1241836
TEST=None

Signed-off-by: John Zhao <john.zhao@intel.com>
Change-Id: I2e1fb5400ff64c6178bb30601896780f8d67b5c6
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44185
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/drivers/usb/ehci_debug.c

@@ -600,7 +600,7 @@ next_debug_port:
 			goto try_next_time;
 	}
 
-	return -10;
+	return ret;
 }
 
 static int dbgp_enabled(void)