cfg80211: fix for duplicate response for driver reg request

As Pavel puts userspace can be stupid and should not
cause kernel crashes. In this case Pavel was able to
find a crash here but unable to reproduce. Either way
lets deal with this.

This should fix:

------------[ cut here ]------------
kernel BUG at /home/proski/src/linux-2.6/net/wireless/reg.c:2132!
Oops: Exception in kernel mode, sig: 5 [#1]
PowerMac
Modules linked in: ath5k ath [last unloaded: scsi_wait_scan]
NIP: c02f3eac LR: c02f3d08 CTR: 00000000
REGS: ef107aa0 TRAP: 0700   Not tainted  (2.6.30-rc8-wl)
MSR: 00029032 <EE,ME,CE,IR,DR>  CR: 88002442  XER: 20000000
TASK = ef84acb0[834] 'crda' THREAD: ef106000
GPR00: ef953840 ef107b50 ef84acb0 ef1380bc 00000006 c035a5c8 ef107b90 c035a5c8
GPR08: 00080005 efb68980 c0445628 ef130004 28002422 10019ce0 10012d3c 00000001
GPR16: 1070b2ac 00000005 48023558 1070b380 4802304c 00000000 ef107ddc c035a5c8
GPR24: ef107b78 c0443350 ef8bcb00 00000005 ef138080 c04a6a70 c04a0000 ef8bcb00
NIP [c02f3eac] set_regdom+0x4c4/0x4ec
LR [c02f3d08] set_regdom+0x320/0x4ec
Call Trace:
[ef107b50] [c02f3d08] set_regdom+0x320/0x4ec (unreliable)
[ef107b70] [c02f9d10] nl80211_set_reg+0x140/0x2d0
[ef107bc0] [c02aa2b8] genl_rcv_msg+0x204/0x228
[ef107c10] [c02a97cc] netlink_rcv_skb+0xe8/0x10c
[ef107c30] [c02aa094] genl_rcv+0x3c/0x5c
[ef107c40] [c02a9050] netlink_unicast+0x308/0x36c
[ef107c80] [c02a92bc] netlink_sendmsg+0x208/0x2f0
[ef107cd0] [c0282048] sock_sendmsg+0xac/0xe4
[ef107db0] [c02822b4] sys_sendmsg+0x234/0x2d8
[ef107f00] [c0283a88] sys_socketcall+0x108/0x258
[ef107f40] [c0012790] ret_from_syscall+0x0/0x38

Signed-off-by: John W. Linville <linville@tuxdriver.com>