soundwire: Update pm runtime last busy for read/write functions

When soundwire read/write operations are called after auto suspend
is triggered, then pm runtime last busy field should be updated
properly for current time. Change updates pm runtime last busy
field after soundwire read/write operations to avoid pm runtime
suspend during read/write operations.

Change-Id: I675f52d96634ed0628ac6d9473337ec531fdba5b
Signed-off-by: Sudheer Papothi <spapothi@codeaurora.org>