[media] v4l: define unit for V4L2_CID_RF_TUNER_BANDWIDTH

Use Hertz as a unit for radio channel bandwidth.

Cc: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>

Documentation/DocBook/media/v4l/controls.xml

@@ -5061,7 +5061,7 @@ by the driver.</entry>
               <entry spanname="descr">Filter(s) on tuner signal path are used to
 filter signal according to receiving party needs. Driver configures filters to
 fulfill desired bandwidth requirement. Used when V4L2_CID_RF_TUNER_BANDWIDTH_AUTO is not
-set. The range and step are driver-specific.</entry>
+set. Unit is in Hz. The range and step are driver-specific.</entry>
             </row>
             <row>
               <entry spanname="id"><constant>V4L2_CID_RF_TUNER_LNA_GAIN_AUTO</constant>&nbsp;</entry>