diag: Initialize USB memory pools even if there are no diag clients

Diag driver was initializing memory pools only if there is at least
one diag client. Make changes to initialize memory pools related to
USB irrespective of diag userspace clients.

Change-Id: I052798a27b86c007bbe1fc49fbf2c2d87f0bf592
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>