p54: Initialize extra_len in p54_tx_80211

This patch fixes a very serious off-by-one bug in
the driver, which could leave the device in an
unresponsive state.

The problem was that the extra_len variable [used to
reserve extra scratch buffer space for the firmware]
was left uninitialized. Because p54_assign_address
later needs the value to reserve additional space,
the resulting frame could be to big for the small
device's memory window and everything would
immediately come to a grinding halt.

Reference: https://bugs.launchpad.net/bugs/722185

Cc: <stable@kernel.org>
Acked-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: Jason Conti <jason.conti@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/p54/txrx.c

@@ -703,7 +703,7 @@ void p54_tx_80211(struct ieee80211_hw *dev, struct sk_buff *skb)
 	struct p54_tx_info *p54info;
 	struct p54_hdr *hdr;
 	struct p54_tx_data *txhdr;
-	unsigned int padding, len, extra_len;
+	unsigned int padding, len, extra_len = 0;
 	int i, j, ridx;
 	u16 hdr_flags = 0, aid = 0;
 	u8 rate, queue = 0, crypt_offset = 0;