power: smb1390: fix NULL pointer de-reference in resume path

Add checks before accessing/de-referencing the pointers.
While at it, add a separate "is_writeable" callback for the slave
SMB1390.

Change-Id: I397f31b526e8d168a7c4bd2831c6dc5fe2fcc83e
Signed-off-by: Ashay Jaiswal <ashayj@codeaurora.org>