rt2x00: zero-out rx_status

In commit 'mac80211: support radiotap vendor namespace RX data'
new fields were added to 'struct ieee80211_rx_status' and those
fileds must be zeroed. However the rt2x00 driver stores driver
specific data in the cb array of the rx skbs, so the fields
might contain garbage and this can cause unexpected behaviour.

The rt2x00 driver from the compat-wireless-2012-12-01
tarball caused the following warning:

  WARNING: at
  /devel/ramips/build_dir/target-mipsel_r2_uClibc-0.9.33.2/linux-ramips_rt305x/
  compat-wireless-2012-12-01/net/mac80211/rx.c:115 ieee80211_rx_irqsafe+0x274/0xbcc
  [mac80211]()
  Modules linked in: dwc_otg ledtrig_usbdev nf_nat_irc
  nf_nat_ftp nf_conntrack_irc nf_conntrack_ftp ipt_MASQUERADE
  iptable_nat nf_nat pppoe xt_conntrack xt_CT xt_NOTRACK iptable_raw
  xt_state nf_conntrack_ipv4 nf_defrag_ipv4 nf_conntrack pppox
  ipt_REJECT xt_TCPMSS xt_comment xt_multiport xt_mac xt_limit
  iptable_mangle iptable_filter ip_tables xt_tcpudp x_tables ppp_async
  ppp_generic slhc rt2800pci(O) rt2800lib(O) rt2x00soc(O) rt2x00pci(O)
  rt2x00lib(O) mac80211(O) usbcore usb_common nls_base crc_itu_t
  crc_ccitt eeprom_93cx6 cfg80211(O) compat(O) arc4 aes_generic
  crypto_blkcipher cryptomgr aead crypto_hash crypto_algapi leds_gpio
  button_hotplug(O) gpio_keys_polled input_polldev input_core
  Call Trace:
  [<801e96b4>] dump_stack+0x8/0x34
  [<80010a9c>] warn_slowpath_common+0x78/0xa4
  [<80010ae0>] warn_slowpath_null+0x18/0x24
  [<80a9710c>] ieee80211_rx_irqsafe+0x274/0xbcc [mac80211]

The patch ensures that each field gets initialized with
zeroes.

Cc: <users@rt2x00.serialmonkey.com>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>