usb: gadget: u_serial: Disable ep before setting port to null to fix the crash caused...

usb: gadget: u_serial: Disable ep before setting port to null to fix the crash caused by port being null

Considering that in some extreme cases, when performing the
unbinding operation, gserial_disconnect has cleared gser->ioport,
which triggers gadget reconfiguration, and then calls gs_read_complete,
resulting in access to a null pointer. Therefore, ep is disabled before
gserial_disconnect sets port to null to prevent this from happening.

Call trace:
 gs_read_complete+0x58/0x240
 usb_gadget_giveback_request+0x40/0x160
 dwc3_remove_requests+0x170/0x484
 dwc3_ep0_out_start+0xb0/0x1d4
 __dwc3_gadget_start+0x25c/0x720
 kretprobe_trampoline.cfi_jt+0x0/0x8
 kretprobe_trampoline.cfi_jt+0x0/0x8
 udc_bind_to_driver+0x1d8/0x300
 usb_gadget_probe_driver+0xa8/0x1dc
 gadget_dev_desc_UDC_store+0x13c/0x188
 configfs_write_iter+0x160/0x1f4
 vfs_write+0x2d0/0x40c
 ksys_write+0x7c/0xf0
 __arm64_sys_write+0x20/0x30
 invoke_syscall+0x60/0x150
 el0_svc_common+0x8c/0xf8
 do_el0_svc+0x28/0xa0
 el0_svc+0x24/0x84

Fixes: c1dca562be8a ("usb gadget: split out serial core")
Cc: stable <stable@kernel.org>
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Lianqin Hu <hulianqin@vivo.com>
Link: https://lore.kernel.org/r/TYUPR06MB621733B5AC690DBDF80A0DCCD2042@TYUPR06MB6217.apcprd06.prod.outlook.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>