arm: fix use of uninitialised value in ff_fft_fixed_init_arm()

When initialising an FFTContext for a plain FFT, mdct_bits is not set
and can contain a garbage value.  Since nbits is always valid and for
MDCT operation is mdct_bits - 2 checking this instead avoids using an
uninitialised value while having the same effect.

Signed-off-by: Mans Rullgard <mans@mansr.com>
