drivers: usb: gadget: add minor fixes

Fix cdev assignments in mbim_set_alt.
Add fifo flush mechanism in mbim_disable.
Fix unlock mechanism in error condition in mbim_write.
Fix unlock mechanism in error condition and also
end point type value in mbim_ioctl.

Change-Id: Ia5432a9008930602a61d87c8efd1f40fb8550877
Signed-off-by: Prasadarao Durvasula <pdurvasu@codeaurora.org>
Signed-off-by: Sivasri Kumar Vanka <sivasri@codeaurora.org>