msm: fb: Null check mfd data

Add appropriate null check for mfd data structure before
accessing it's members. This prevents null pointer accesses
in the driver.

Change-Id: I5aae937f839e4f6ed8d5c5a4b54ab45f0ff07332
Signed-off-by: Krishna Chaitanya Parimi <cparimi@codeaurora.org>
Signed-off-by: Neti, Ravi Kumar <ravineti@codeaurora.org>