[media] dvb-frontend: Use boottime

Wall time obtained from ktime_get_real is susceptible to sudden jumps due to
user setting the time or due to NTP.  Boot time is constantly increasing time
better suited for comparing two timestamps.

[mchehab@osg.samsung.com: fix trivial merge conflicts]
Signed-off-by: Abhilash Jindal <klock.android@gmail.com>

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