mfd: don't use memzero

For it doesn't exist on i386.

Cc: Ian Molton <spyro@f2s.com>
Cc: Dmitry Baryshkov <dbaryshkov@gmail.com>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>