fpga: altera-cvp: Fix registration for CvP incapable devices

The probe function needs to verify the CvP enable bit in order to
properly determine if FPGA Manager functionality can be safely
enabled.

Fixes: 34d1dc17 ("fpga manager: Add Altera CvP driver")
Signed-off-by: Andreas Puhm <puhm@oregano.at>
Signed-off-by: Anatolij Gustschin <agust@denx.de>
Reviewed-by: Moritz Fischer <mdf@kernel.org>
Acked-by: Alan Tull <atull@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>