greybus: loopback: convert sample report interface to debugfs

Greg previously suggested switching over to debugfs instead of a char
interface to report raw samples to user-space. At the time we agreed not
to go for that change. However later patches in this series are made
simpler if debugfs is used instead of /dev, so it makes sense to make the
conversion now for that reason. This patch removes the char interface and
replaces it with a debugfs interface. Raw samples will be acquired from
/sys/kernel/debug/gb_loopback/raw_latency_endo0:x:y:z:w where

x = <module-id>
y = <interface-id>
z = <bundle-id>
w = <cport-number>

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>