cfg80211: errno.h: define ERFKILL

Commit 1f87f7d3 (cfg80211: add rfkill support) added ERFKILL
to asm-generic/errno.h, but alpha, mips, parisc and sparc use
their own numbering scheme and do not include asm-generic/errno.h.
We need to add definition of ERFKILL for them.

Signed-off-by: Alexander Beregalov <a.beregalov@gmail.com>
Acked-by: Ralf Baechle <ralf@linux-mips.org>
Acked-by: Kyle McMartin <kyle@mcmartin.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>