firmware: provide helper for FW_OPT_USERHELPER

The macro FW_OPT_USERHELPER is only currently defined when
CONFIG_FW_LOADER_USER_HELPER is set. This is handled via an
ifdef around CONFIG_FW_LOADER_USER_HELPER. This makes reading
and understanding use FW_OPT_USERHELPER a bit convoluted.

Instead wrap the functionality implemented behind
CONFIG_FW_LOADER_USER_HELPER as we typically do in the
kernel.

Now when CONFIG_FW_LOADER_USER_HELPER is *not set*, then
simply the helper fw_sysfs_fallback() will not do anything.

Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>