sound: usb: Flush cache explicitly after mapping buffers

For USB Audio accelerated path it is must to explicitly flush cache
before giving mapped event ring, transfer ring and data buffer to
ADSP firmware as allocated memory is cacheable whereas ADSP
subsystem doesn't use cacheable memory. This helps to have coherent
memory seen from ADSP firmware.

Change-Id: I9aff88cdba18acb1cc181beee16e9db2ec167b88
Signed-off-by: Mayank Rana <mrana@codeaurora.org>