fpga: spartan2: Use logging feature instead of FPGA_DEBUG

Instead of using DEBUG or LOG_DEBUG the driver still had its own
definition for debug output.

Signed-off-by: Alexander Dahl <ada@thorsis.com>
Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/20221007122003.11239-9-ada@thorsis.com