[media] dib8000: remove a double call for dib8000_get_symbol_duration()

The symbol duration was already obtained at CT_DEMOD_START.
No need to do it again at CT_DEMOD_STEP_3.

Acked-By: Patrick Boettcher <pboettcher@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>