x86/synth_filter: Revert the switch to float ops with SSE2

This reverts the changes 64672098361361cd15d37e36f747ab44de5b80ca
and 68c3ed936a76c3ff7738f602fa90237ac7e3ce08 did to the SSE2 version,
which generated a hit of about 5 cycles.

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>