USB: mass_storage: Fix mass storage LUN creation with Android gadget

Mass storage driver has changed in terms of its usage due to moving
with configfs functionality. This change adds minimum LUN creation
functionality in order to work with Android gadget driver. It also
adds support to create sysfs entry to access created LUN.

Android gadget driver is not having any requirement to use more than
one instance of mass storage driver as one instance of mass storage
driver supports 8 LUNS. Hence move to creating and using one instance
of USB mass storage driver.

Change-Id: I1c2655da1cac069abce679ca3b969e264edf0b09
Signed-off-by: Mayank Rana <mrana@codeaurora.org>
[jackp@codeaurora.org: Also convert android.c to use the
 USB_FUNCTION API of the mass_storage driver]
Signed-off-by: Jack Pham <jackp@codeaurora.org>