avcodec/kbdwin: Remove low precision intermediate in ff_kbd_window_init_fixed()

Previously floats where scaled up to 32bit int, but floats do not
have 32bits in their mantisse so a quarter of the bits where nonsense.

It seems no fate test is affected by this change, which is interresting

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
