iwlegacy: fix up brokenness caused by cleanup

Patch 7f1f9742 "iwlegacy: refactor iwl4965_mac_channel_switch"
reversed a test and also it changed a spin_lock_irq() to
spin_lock().  I assume the spin_lock change was a mistake because
the unlock was left as spin_unlock_irq().

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>