ssbi: Add null pointer check

Add null pointer check for ssbi_read and ssbi_write

Change-Id: I00d0547e5392d2dc079610f31e2e9d7e1ed4e199
Acked-by: Edward Rodrigues <erodrigu@qti.qualcomm.com>
Signed-off-by: Venkatesh Yadav Abbarapu <quicvenkat@codeaurora.org>