drx-j: use ber_count var

drivers/media/dvb-frontends/drx39xyj/drxj.c: In function 'ctrl_get_qam_sig_quality':
drivers/media/dvb-frontends/drx39xyj/drxj.c:9468:6: warning: variable 'ber_cnt' set but not used [-Wunused-but-set-variable]
  u32 ber_cnt = 0; /* BER count */
      ^

By reading the comment, it is said that BER should be calculated as:
	qam_pre_rs_ber = frac_times1e6( ber_cnt, rs_bit_cnt );

Also, it makes sense to take the mantissa into account, so fix the
code to do what's commented.

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>