drivers: qcom: ultrasound: Lock async driver calls

Adds lock to ioctl and other external calls to driver.
Adds missing null check in __usf_set_stream_param.

Change-Id: I142f31c6bb46d6a394ad012077e1703875a120ad
Signed-off-by: Sudheer Papothi <spapothi@codeaurora.org>