Set explicit timeout on USB control transfer

This attempts to solve the intermittent issues with serial number
showing up as all question marks when issuing "fastboot devices"
diff --git a/fastboot/usb_linux.c b/fastboot/usb_linux.c
index 2ce53eb..78b7b98 100644
--- a/fastboot/usb_linux.c
+++ b/fastboot/usb_linux.c
@@ -137,6 +137,7 @@
         ctrl.wIndex = 0;
         ctrl.wLength = sizeof(buffer);
         ctrl.data = buffer;
+	ctrl.timeout = 50;
 
         result = ioctl(fd, USBDEVFS_CONTROL, &ctrl);
         if (result > 0) {