libavcodec/iff: Use unsigned to avoid undefined behaviour

libavcodec/iff: Use unsigned to avoid undefined behaviour

The initialization of the uint32_t plane32_lut matrix uses left shifts
of the form 1 << plane; plane can be as big as 31 which means that this
is undefined behaviour as 1 will be simply an int. So make it unsigned
to avoid this.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>