[media] rc: fix up and genericize some time unit conversions

The ene_ir driver was using a private define of MS_TO_NS, which is meant
to be microseconds to nanoseconds. The mceusb driver copied it,
intending to use is a milliseconds to microseconds. Lets move the
defines to a common location, expand and standardize them a touch, so
that we now have:

  MS_TO_NS - milliseconds to nanoseconds
  MS_TO_US - milliseconds to microseconds
  US_TO_NS - microseconds to nanoseconds

Reported-by: David Härdeman <david@hardeman.nu>
CC: Maxim Levitsky <maximlevitsky@gmail.com>
Signed-off-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>