msm: ultrasound: Various static analysis fixes

Remove potential null dereference.
Remove dead code from impossible error check.
Set upper bound for user-provided buffer size.

Change-Id: I212f3edfa31e9168d11e0782da7f3fd8c2b98c5d
Signed-off-by: Vidyakumar Athota <vathota@codeaurora.org>