usb: diag: Do not access all the channels from set_alt handler

Access contexts of only that diag channel for which set_alt is
called. Otherwise, it might result in driver wrongly trying to
access a channel which is not included in the composition.

Change-Id: I3380d710bb372d0e4377300f44322c0866be1bcc
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>