We might kill TX or RX urb during rt2x00usb_flush_entry(), what can
cause anchor list corruption like shown below:

[ 2074.035633] WARNING: CPU: 2 PID: 14480 at lib/list_debug.c:33 __list_add+0xac/0xc0
[ 2074.035634] list_add corruption. prev->next should be next (ffff88020f362c28), but was dead000000000100. (prev=ffff8801d161bb70).
<snip>
[ 2074.035670] Call Trace:
[ 2074.035672]  [<ffffffff813bde47>] dump_stack+0x63/0x8c
[ 2074.035674]  [<ffffffff810a2231>] __warn+0xd1/0xf0
[ 2074.035676]  [<ffffffff810a22af>] warn_slowpath_fmt+0x5f/0x80
[ 2074.035678]  [<ffffffffa073855d>] ? rt2x00usb_register_write_lock+0x3d/0x60 [rt2800usb]
[ 2074.035679]  [<ffffffff813dbe4c>] __list_add+0xac/0xc0
[ 2074.035681]  [<ffffffff81591c6c>] usb_anchor_urb+0x4c/0xa0
[ 2074.035683]  [<ffffffffa07322af>] rt2x00usb_kick_rx_entry+0xaf/0x100 [rt2x00usb]
[ 2074.035684]  [<ffffffffa0732322>] rt2x00usb_clear_entry+0x22/0x30 [rt2x00usb]

To fix do not anchor TX and RX urb's, it is not needed as during
shutdown we kill those urbs in rt2x00usb_free_entries().

Cc: Vishal Thanki <vishalthanki@gmail.com>
Fixes: 8b4c0009 ("rt2x00usb: Use usb anchor to manage URB")
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>