Replace impossible BUG() in probe with a proper return.  This can be
triggered only in case of a clear bug (e.g. adding broken half-support
for new S2MPSXX flavor) but BUG() is discouraged and the boot process
can actually try to continue.
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>