mwifiex: use sizeof(array) to print_hex_dump_bytes

DBG_CMD_NUM is the number of commands, not the actual bytes of
data for printing.

Also remove the duplicated DBG_CMD_NUM definition.

Reported-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>