mips/acelp_filters: fix incorrect register constraint

Change register constraint on the v variable from = to +. This was causing GCC
to think that the v variable was never read and therefore not initialize it.

This fixes about 20 fate failures on mips64el.

Signed-off-by: James Cowgill <james410@cowgill.org.uk>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
