spcom: check error from ion_import_dma_buf()

Check ion_import_dma_buf() returned handle is not NULL or error code,
by using IS_ERR_OR_NULL() macro.
Check channel name is null terminated on create channel command.
Check minimal command size from user space.
Fix spelling.

Change-Id: I81631c1844982952498eaf2bff84ff90578720b2
Signed-off-by: Amir Samuelov <amirs@codeaurora.org>