xtensa: fix incorrect memset

Addresses: https://bugzilla.kernel.org/show_bug.cgi?id=43871



Reported-by:  <dcb314@hotmail.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Chris Zankel <chris@zankel.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>