mac80211: Do not use sizeof() on pointer type

The rate_control_cap_mask() function takes a parameter mcs_mask, which
GCC will take to be u8 * even though it was declared with a fixed size.
This causes the following warning:

	net/mac80211/rate.c: In function 'rate_control_cap_mask':
	net/mac80211/rate.c:719:25: warning: 'sizeof' on array function parameter 'mcs_mask' will return size of 'u8 * {aka unsigned char *}' [-Wsizeof-array-argument]
	   for (i = 0; i < sizeof(mcs_mask); i++)
	                         ^
	net/mac80211/rate.c:684:10: note: declared here
	       u8 mcs_mask[IEEE80211_HT_MCS_MASK_LEN],
	          ^

This can be easily fixed by using the IEEE80211_HT_MCS_MASK_LEN directly
within the loop condition.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>