fpga: altera-cvp: remove DRIVER_ATTR() usage

It's better to be explicit and use the DRIVER_ATTR_RW() macro when
defining a driver's sysfs file.

This is part of a series to drop DRIVER_ATTR() from the tree entirely.

Cc: linux-fpga@vger.kernel.org
Reviewed-by: Moritz Fischer <mdf@kernel.org>
Acked-by: Alan Tull <atull@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>