staging: most: remove unnecessary field initialization

Since conf->extra_len has already been reset in most_start_channel() when
function hdm_configure_channel() gets called, it can safely be removed
here.

Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>