Remove '=' from 'if CONFIG_XEN_CTRL_INTERFACE_VERSION <= 41500'.
Because xendevicemodel_set_irq_level() was introduced in 4.15 version.

Also, update xendevicemodel_set_irq_level() to return -1 for older versions.

Signed-off-by: Vikram Garhwal <vikram.garhwal@amd.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>