lib: enable standby mode of e4k and r820t on exit

Since the R820T is a power hog and gets quite hot,
this makes sense, especially when being battery-
powered.

Signed-off-by: Steve Markgraf <steve@steve-m.de>
exit if our usb device disappears
Merge branch 'master' of git://git.osmocom.org/rtl-sdr