gmr1_rx: Don't count the borders when computing energy
gmr1_rx: Don't count the borders when computing energy

Since the 'burst' vector often include some margin, the center
will be more representative of the real average energy.

Signed-off-by: Sylvain Munaut <349f318521b45672c77207abf1a5ed60eb011db1@246tNt.com>

marcelmaatkamp/gnuradio-osmocom-gmr,marcelmaatkamp/docker-gnuradio-osmocom-gmr,marcelmaatkamp/docker-gnuradio-osmocom-gmr,marcelmaatkamp/gnuradio-osmocom-gmr,marcelmaatkamp/gnuradio-osmocom-gmr,marcelmaatkamp/docker-gnuradio-osmocom-gmr