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 <benoit.thebaudeau.dev@gmail.com>