x86, mce: Fix set_trigger() accessor

Fix the condition checking the result of strchr() (which previously
could result in an oops), and make the function return the number of
bytes actively used.

[ Impact: fix oops ]

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Cc: Andi Kleen <andi@firstfloor.org>
LKML-Reference: <4A5F04B7020000780000AB59@vpn.id2.novell.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>