bif: bif-core: fall back on time-invariant transactions for read and write

Fall back and use time-invariant individual transactions for BIF
slave register read and write operations if high level read or
write controller callback functions fail.

Change-Id: Ieaf479717383719dc1f5b94cc4be72f2e8901127
Signed-off-by: David Collins <collinsd@codeaurora.org>