arm/neon: dsputil: use correct size specifiers on vld1/vst1

Change the size specifiers to match the actual element sizes
of the data.  This makes no practical difference with strict
alignment checking disabled (the default) other than somewhat
documenting the code.  With strict alignment checking on, it
avoids trapping the unaligned loads.
Signed-off-by: Mans Rullgard <mans@mansr.com>