libusbhost: Fix issue of multiple inotify event at same time.

libusbhost can handle multiple inotify events at same time on Kit kat.
In original implementation, the function-level ret is assigned with
length of read and used for while loop.
if want to add a new watch for subdirectory,inotify_add_watch will
update the function-level ret with watch descriptor.
It impact on while loop to do exit.

Add local result to avoid it.

Change-Id: I101041ccfd54eaac14969a45e90ab2e49bdff578
Signed-off-by: Bo Huang <bo.b.huang@intel.com>