msm: synx: fix secure key generation

They secure key generated to authenticate synx object
sharing across processes is being overwitten on every
export function. This results in key mismatch.
Change fixes this issue by creating the key on first
invocation and return the same key for all successive
export calls till the synx object is destroyed.

Change-Id: Id7f4c1e6b2b01749955a172de6a41640f9db26d6
Signed-off-by: Sumukh Hallymysore Ravindra <shallymy@codeaurora.org>