xtensa: fix memscan()

Defining memscan() as memchr() is wrong, because the return values of
memscan() and memchr() are different when the character is not found.  So
use the generic memscan() implementation to fix this.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Chris Zankel <chris@zankel.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>