ath: wil6210: Fix build error

Building driver wil6210 in 3.10 and 3.11 kernels yields the following errors:

  CC [M]  drivers/net/wireless/ath/wil6210/debugfs.o
drivers/net/wireless/ath/wil6210/debugfs.c: In function 'wil_print_ring':
drivers/net/wireless/ath/wil6210/debugfs.c:163:11: error: pointer targets in passing argument 5 of 'hex_dump_to_buffer' differ in signedness [-Werror=pointer-sign]
           false);
           ^
In file included from include/linux/kernel.h:13:0,
                 from include/linux/cache.h:4,
                 from include/linux/time.h:4,
                 from include/linux/stat.h:18,
                 from include/linux/module.h:10,
                 from drivers/net/wireless/ath/wil6210/debugfs.c:17:
include/linux/printk.h:361:13: note: expected 'char *' but argument is of type 'unsigned char *'
 extern void hex_dump_to_buffer(const void *buf, size_t len,
             ^
drivers/net/wireless/ath/wil6210/debugfs.c: In function 'wil_txdesc_debugfs_show':
drivers/net/wireless/ath/wil6210/debugfs.c:429:10: error: pointer targets in passing argument 5 of 'hex_dump_to_buffer' differ in signedness [-Werror=pointer-sign]
          sizeof(printbuf), false);
          ^
In file included from include/linux/kernel.h:13:0,
                 from include/linux/cache.h:4,
                 from include/linux/time.h:4,
                 from include/linux/stat.h:18,
                 from include/linux/module.h:10,
                 from drivers/net/wireless/ath/wil6210/debugfs.c:17:
include/linux/printk.h:361:13: note: expected 'char *' but argument is of type 'unsigned char *'
 extern void hex_dump_to_buffer(const void *buf, size_t len,
             ^
cc1: all warnings being treated as errors
make[5]: *** [drivers/net/wireless/ath/wil6210/debugfs.o] Error 1
make[4]: *** [drivers/net/wireless/ath/wil6210] Error 2
make[3]: *** [drivers/net/wireless/ath] Error 2
make[2]: *** [drivers/net/wireless] Error 2
make[1]: *** [drivers/net] Error 2
make: *** [drivers] Error 2

These errors are fixed by changing the type of the buffer from "unsigned char *" to "char *".

Reported-by: Thomas Fjellstrom <thomas@fjellstrom.ca>
Tested-by: Thomas Fjellstrom <thomas@fjellstrom.ca>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Stable <stable@vger.kernel.org>  [3.10]
Cc: Thomas Fjellstrom <thomas@fjellstrom.ca>
Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>