Fix integer handling issues handling issue reported by Coverity:

  hw/ssi/npcm7xx_fiu.c: 162 in npcm7xx_fiu_flash_read()
  >>>     CID 1432730:  Integer handling issues  (NEGATIVE_RETURNS)
  >>>     "npcm7xx_fiu_cs_index(fiu, f)" is passed to a parameter that cannot be negative.
  162         npcm7xx_fiu_select(fiu, npcm7xx_fiu_cs_index(fiu, f));

  hw/ssi/npcm7xx_fiu.c: 221 in npcm7xx_fiu_flash_write()
  218         cs_id = npcm7xx_fiu_cs_index(fiu, f);
  219         trace_npcm7xx_fiu_flash_write(DEVICE(fiu)->canonical_path, cs_id, addr,
  220                                       size, v);
  >>>     CID 1432729:  Integer handling issues  (NEGATIVE_RETURNS)
  >>>     "cs_id" is passed to a parameter that cannot be negative.
  221         npcm7xx_fiu_select(fiu, cs_id);

Since the index of the flash can not be negative, return an
unsigned type.

Reported-by: Coverity (CID 1432729 & 1432730: NEGATIVE_RETURNS)
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Havard Skinnemoen <hskinnemoen@google.com>
Message-id: 20200919132435.310527-1-f4bug@amsat.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>