wigig_sensing: handle SYS_ASSERT corner cases

Return -ENODEV in case a change_mode command is issued when the system
is in SYS_ASSERT state. This guarantees that the user space application
is aware of the bad state and waits for the FW_READY event.

Change-Id: If26dc115078a3cd1af5d7efa93225019c1105184
Signed-off-by: Gidon Studinski <gidons@codeaurora.org>