soc: ti: knav: Add a NULL pointer check for kdev in knav_pool_create

knav_pool_create is an exported function. In the event of a call
before knav_queue_probe, we encounter a NULL pointer dereference
in the following line. Hence return -EPROBE_DEFER to the caller till
the kdev pointer is non-NULL.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>