spmi: Inline a function to remove compilation error

Compilation error is seen in spmi_dfs_add_controller
function when debug fs is disabled.
In this patch this function is defined as static
inline to remove this error.

CRs-Fixed: 555022
Change-Id: I66ac9ca9eeb1e25c68c9deff665258f4e036a678
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>