If no channels are enabled when we run generic_buffer on a
device, add a command-line option to just enable all of them,
run the sampling and disable them all again afterwards.

This is extremely useful when I'm low-level testing my
sensors with interrupts and triggers, sample session:

root@Ux500:/ lsiio
Device 000: lsm303dlh_accel
Device 001: lis331dl_accel
Device 002: l3g4200d
Device 003: lsm303dlh_magn
Device 004: lps001wp
Trigger 000: lsm303dlh_accel-trigger
Trigger 001: lis331dl_accel-trigger
Trigger 002: l3g4200d-trigger

root@Ux500:/ generic_buffer -a -c 10 -n l3g4200d
iio device number being used is 2
iio trigger number being used is 2
No channels are enabled, enabling all channels
Enabling: in_anglvel_x_en
Enabling: in_anglvel_y_en
Enabling: in_anglvel_z_en
Enabling: in_timestamp_en
/sys/bus/iio/devices/iio:device2 l3g4200d-trigger
-3.593664 -0.713133 4.870143 946684863662292480
3.225546 0.867357 -4.945878 946684863671875000
-0.676413 0.127296 0.106641 946684863681488037
-0.661113 0.110160 0.128826 946684863690673828
-0.664173 0.113067 0.123471 946684863700683593
-0.664938 0.109395 0.124848 946684863710144042
-0.664173 0.110619 0.130203 946684863719512939
-0.666162 0.111231 0.132651 946684863729125976
-0.668610 0.111690 0.130662 946684863738739013
-0.660501 0.110466 0.131733 946684863748565673
Disabling: in_anglvel_x_en
Disabling: in_anglvel_y_en
Disabling: in_anglvel_z_en
Disabling: in_timestamp_en

Pure awesomeness. If some channels have been enabled through
scripts or manual interaction, nothing happens.

Cc: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Acked-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>