mfd: syscon: Return ENOTSUPP instead of ENOSYS when disabled

When CONFIG_MFD_SYSCON is disabled, have the function stubs return
ENOTSUPP to indicate the syscon functionality is not available.
There are currently no callers that depend on the ENOSYS return value.

This patchfixes a checkpatch warning:
    WARNING: ENOSYS means 'invalid syscall nr' and nothing else

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>