usb: gadget: gadgetfs: Initialize CHIP to NULL before UDC probe

Otherwise the value from the last probe would be retained that possibly is
freed since (the UDC is removed) and therefore no longer relevant. Reproducible
with the dummy UDC:

  modprobe dummy_hcd
  mount -t gadgetfs gadgetfs /dev/gadget
  umount /dev/gadget
  rmmod dummy_hcd
  mount -t gadgetfs gadgetfs /dev/gadget

BUG: unable to handle kernel paging request at ffffffffa066fd9d
Call Trace:
 [<ffffffff811d0cd2>] ? d_alloc_name+0x22/0x50
 [<ffffffff812b74dc>] ? selinux_d_instantiate+0x1c/0x20
 [<ffffffffa067d687>] gadgetfs_create_file+0x27/0xa0 [gadgetfs]
 [<ffffffffa067da70>] ? setup_req.isra.4+0x80/0x80 [gadgetfs]
 [<ffffffffa067dbac>] gadgetfs_fill_super+0x13c/0x180 [gadgetfs]
 [<ffffffff811bc832>] mount_single+0x92/0xc0
 [<ffffffffa067d0f8>] gadgetfs_mount+0x18/0x20 [gadgetfs]
 [<ffffffff811bc8f9>] mount_fs+0x39/0x1b0
 [<ffffffff8116b220>] ? __alloc_percpu+0x10/0x20
 [<ffffffff811d6da3>] vfs_kern_mount+0x63/0xf0
 [<ffffffff811d93be>] do_mount+0x23e/0xac0
 [<ffffffff811660eb>] ? strndup_user+0x4b/0xf0
 [<ffffffff811d9f63>] SyS_mount+0x83/0xc0
 [<ffffffff81695b69>] system_call_fastpath+0x16/0x1b

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Felipe Balbi <balbi@ti.com>