qcom: ssr: Fix possible overflow when copying firmware name

Array overflow can occur in firmware_name_store(), if the variable
buf contains the string larger than size of subsys->desc->fw_name

Change-Id: Ice39d7a1eb0b5f53125cc5d528021a99b9f7ff90
Signed-off-by: Srinivasarao P <spathi@codeaurora.org>
Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>