swresample/soxr: fix invalid use of linear_interp

give very bad quality for soxr resampler.
linear_interp is intended for  using linear interpolation
between filter bank so quality will be better.

i guess this is misunderstood as 'do not use filter bank,
but directly interpolate linearly between samples'.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Muhammad Faiz <mfcc64@gmail.com>