firmware: xilinx: Use explicit values for all enum values

Based on discussion at
https://lore.kernel.org/r/20200318125003.GA2727094@kroah.com we got
recommendation to use explicit values for all enum values.
The patch is following this recommendation.
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/daeb67ded45d8a8f6a96717d1fb9c84439dd2ae8.1612361627.git.michal.simek@xilinx.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>