fddi/skfp: Avoid the use of one-element array

One-element arrays are being deprecated[1]. Replace the one-element array
with a simple object of type u_char: 'u_char rm_pad1'[2], once it seems
this is just a placeholder for padding.

[1] https://www.kernel.org/doc/html/v5.9-rc1/process/deprecated.html#zero-length-and-one-element-arrays
[2] https://github.com/KSPP/linux/issues/86



Built-tested-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/lkml/5f72c23f.%2FkPBWcZBu+W6HKH4%25lkp@intel.com/


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>