b43legacy: Remove pointless cond_resched() wrapper

cond_resched() can be used unconditionally. If CONFIG_PREEMPT is set, it
becomes a NOP scheduler wise.

Also the B43_BUG_ON() in that wrapper is a homebrewn variant of
__might_sleep() which is part of cond_resched() already.

Remove the cruft and invoke cond_resched() directly.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Larry Finger <Larry.Finger@lwfinger.net>
Tested-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>