usb_serial.c: Avoid inline error

We've got with -Winline:
usb_serial.c:761:13: error: inlining failed in call to 'usb_wait_in_ready': call is unlikely and code size would grow [-Werror=inline]

We want to inline this in the USB ISR. So let's force it and prevent the error.

usb_serial.c

@@ -757,6 +757,7 @@ ISR(USB_GEN_vect)
 
 
 // Misc functions to wait for ready and send/receive packets
+static void usb_wait_in_ready(void) __attribute__ ((always_inline));
 inline void usb_wait_in_ready(void)
 {
 	while (!(UEINTX & (1<<TXINI))) ;