spcom: open channel return EBUSY when channel in use

The char device /dev/sp_kernel is used for loading SP application and
creating new channel char device, therefore might be temporary busy.
Return EBUSY error to user system call open() so it can retry.

Change-Id: I230577fe6b03ac7afd7175f72f9a3fa80db5a524
Signed-off-by: Amir Samuelov <amirs@codeaurora.org>