greybus: encapsulate URB status checking

Define a new common function check_urb_status() that looks at the
status in a completed URB (containing incoming data) and issues
warnings in a consistent way.  It returns -EAGAIN to signal an
unrecognized status was seen, so the caller can ignore it and
re-post the URB to receive the next incoming data.  This
consolidates three blocks of code into one.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>