cc2538: Fix GCC 5 warning

Fix the following warning issued by GCC ARM Embedded 5-2015-q4-major:

  ../../cpu/cc2538/dev/udma.c: In function 'udma_init':
  ../../cpu/cc2538/dev/udma.c:59:10: warning: passing argument 1 of 'memset' discards 'volatile' qualifier from pointer target type [-Wdiscarded-array-qualifiers]
     memset(&channel_config, 0, sizeof(channel_config));
            ^
  In file included from <toolchain-path>/arm-none-eabi/include/string.h:10:0,
                   from ../../platform/cc2538dk/./contiki-conf.h:12,
                   from ../../cpu/cc2538/dev/udma.c:38:
  <toolchain-path>/arm-none-eabi/include/string.h:25:7: note: expected 'void *' but argument is of type 'volatile struct channel_ctrl (*)[4]'
   _PTR  _EXFUN(memset,(_PTR, int, size_t));
         ^

Signed-off-by: Benoît Thébaudeau <[email protected]>

cpu/cc2538/dev/udma.c

@@ -56,7 +56,7 @@ static volatile struct channel_ctrl channel_config[UDMA_CONF_MAX_CHANNEL + 1]
 void
 udma_init()
 {
-  memset(&channel_config, 0, sizeof(channel_config));
+  memset((void *)&channel_config, 0, sizeof(channel_config));
 
   REG(UDMA_CFG) = UDMA_CFG_MASTEN;