[B43]: Don't lock irq_lock in debugfs txpower adjust

It's not required and the txpower adjustment must not be in atomic.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>