net/wireless: use ARRAY_SIZE macro in radiotap.c

Replace sizeof(rtap_namespace_sizes) / sizeof(rtap_namespace_sizes[0])
with ARRAY_SIZE(rtap_namespace_sizes) in net/wireless/radiotap.c

Signed-off-by: Nikitas Angelinas <nikitasangelinas@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>