Currently the link_state is uninitialized and the default value is 0(U0)
before the first time we start the udc, and after we start the udc then
 stop the udc, the link_state will be undefined.
We may have the following warnings if we start the udc again with
an undefined link_state:

WARNING: CPU: 0 PID: 327 at drivers/usb/dwc3/gadget.c:294 dwc3_send_gadget_ep_cmd+0x304/0x308
dwc3 100e0000.hidwc3_0: wakeup failed --> -22
[...]
Call Trace:
[<c010f270>] (unwind_backtrace) from [<c010b3d8>] (show_stack+0x10/0x14)
[<c010b3d8>] (show_stack) from [<c034a4dc>] (dump_stack+0x84/0x98)
[<c034a4dc>] (dump_stack) from [<c0118000>] (__warn+0xe8/0x100)
[<c0118000>] (__warn) from [<c0118050>](warn_slowpath_fmt+0x38/0x48)
[<c0118050>] (warn_slowpath_fmt) from [<c0442ec0>](dwc3_send_gadget_ep_cmd+0x304/0x308)
[<c0442ec0>] (dwc3_send_gadget_ep_cmd) from [<c0445e68>](dwc3_ep0_start_trans+0x48/0xf4)
[<c0445e68>] (dwc3_ep0_start_trans) from [<c0446750>](dwc3_ep0_out_start+0x64/0x80)
[<c0446750>] (dwc3_ep0_out_start) from [<c04451c0>](__dwc3_gadget_start+0x1e0/0x278)
[<c04451c0>] (__dwc3_gadget_start) from [<c04452e0>](dwc3_gadget_start+0x88/0x10c)
[<c04452e0>] (dwc3_gadget_start) from [<c045ee54>](udc_bind_to_driver+0x88/0xbc)
[<c045ee54>] (udc_bind_to_driver) from [<c045f29c>](usb_gadget_probe_driver+0xf8/0x140)
[<c045f29c>] (usb_gadget_probe_driver) from [<bf005424>](gadget_dev_desc_UDC_store+0xac/0xc4 [libcomposite])
[<bf005424>] (gadget_dev_desc_UDC_store [libcomposite]) from[<c023d8e0>] (configfs_write_file+0xd4/0x160)
[<c023d8e0>] (configfs_write_file) from [<c01d51e8>] (__vfs_write+0x1c/0x114)
[<c01d51e8>] (__vfs_write) from [<c01d5ff4>] (vfs_write+0xa4/0x168)
[<c01d5ff4>] (vfs_write) from [<c01d6d40>] (SyS_write+0x3c/0x90)
[<c01d6d40>] (SyS_write) from [<c0107400>] (ret_fast_syscall+0x0/0x3c)
Signed-off-by: Zeng Tao <prime.zeng@hisilicon.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>