swscale: arm: fix NEON hscale init

swscale: arm: fix NEON hscale init

The NEON hscale function only supports X8 filter sizes and should only
be selected when these are being used. At the moment filterAlign is
set to 8 but in the future when extra NEON assembly for specific sizes is
added they will need to have checks here too.

The immediate usecase for this change is making the hscale checkasm
test easier and without NEON specific edge-cases (x86 already has these
guards).

This applies the same fix from 718c8f9aa59751bb490e2688acf2b5cb68fd5ad1
on the 32 bit arm version of the function, fixing fate-checkasm-sw_scale
there.

Signed-off-by: Martin Storsjö <martin@martin.st>