[media] drxk_hard: don't re-implement log10

X-Patchwork-Delegate: mchehab@redhat.com
Log10 routine is already defined at dvb_math.h and provides a good
enough approximation for 100 x log10(). So, instead of reinventing
the wheel, use the already existing function.
While here, don't use CamelCase on the function name.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>