Update usb_read_write.py

Hi,
I also ran into this issue:
canonical#1587

I tried this change on my DUT and it works for mediacard test.

checkbox-support/checkbox_support/scripts/usb_read_write.py

@@ -170,6 +170,8 @@ def mount_usb_storage(partition):
         # use pipe so I could hide message like
         # "umount: /tmp/tmpjzwb6lys: not mounted"
         subprocess.call(["umount", FOLDER_TO_MOUNT], stderr=subprocess.PIPE)
+        # umount device_to_mount such as "umount /dev/mmcblk0p1" to avoid when the storage type is ntfs, which could not be mounted twice.
+        subprocess.call(["umount", device_to_mount], stderr=subprocess.PIPE)
         # mount the target device/partition
         # if the return code of the shell command is non-zero,
         # means something wrong.