aesni: Remove misleading array bounds for aesni_decryt_ecb.

All the other functions used pointers for from/to instead of
fixed-size array parameters. More importantly, this function can
accept pointers to buffers of multiple blocks, not just a single
block.

Reported by: GCC -Warray-parameter
Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D37547

(cherry picked from commit d256a06fe8bda8120a3e22434c53fdef076001d3)