alpha: use BUG_ON where possible

Just use BUG_ON() instead of constructions such as:

	if (...)
		BUG()

A simplified version of the semantic patch that makes this transformation
is as follows: (http://coccinelle.lip6.fr/

)

// <smpl>
@@
expression e;
@@
- if (e) BUG();
+ BUG_ON(e);
// </smpl>

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>