fpga: Avoid ignored-qualifiers warning

fpga: Avoid ignored-qualifiers warning

Fixes annoying warnings of the following type when built with W=1 (for
each file including fpga.h):

      CC      drivers/fpga/fpga.o
    In file included from /mnt/data/adahl/src/u-boot/include/xilinx.h:7,
                     from /mnt/data/adahl/src/u-boot/drivers/fpga/fpga.c:10:
    /mnt/data/adahl/src/u-boot/include/fpga.h:61:1: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
       61 | const fpga_desc *const fpga_get_desc(int devnum);
          | ^~~~~
    /mnt/data/adahl/src/u-boot/include/fpga.h:81:1: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
       81 | const fpga_desc *const fpga_validate(int devnum, const void *buf,
          | ^~~~~
    /mnt/data/adahl/src/u-boot/drivers/fpga/fpga.c:36:1: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
       36 | const fpga_desc *const fpga_get_desc(int devnum)
          | ^~~~~
    /mnt/data/adahl/src/u-boot/drivers/fpga/fpga.c:53:1: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
       53 | const fpga_desc *const fpga_validate(int devnum, const void *buf,
          | ^~~~~

Do some type and cast cleanup on that fpga_desc type while at it.

Link: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wignored-qualifiers
Signed-off-by: Alexander Dahl <ada@thorsis.com>
Link: https://lore.kernel.org/r/20250121162213.1477506-1-ada@thorsis.com
Signed-off-by: Michal Simek <michal.simek@amd.com>