mwifiex: correction in number of bitrates

In recent commit "mwifiex: correct bitrates advertised..", we have
removed 22Mbps and 72Mbps bitrates from supported bitrate array.
It means number of bitrates has reduced from 14 to 12.

Initialize ".n_bitrates" to array size instead of hardcoding it.
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/mwifiex/cfg80211.c

@@ -646,7 +646,7 @@ static struct ieee80211_supported_band mwifiex_band_2ghz = {
 	.channels = mwifiex_channels_2ghz,
 	.n_channels = ARRAY_SIZE(mwifiex_channels_2ghz),
 	.bitrates = mwifiex_rates,
-	.n_bitrates = 14,
+	.n_bitrates = ARRAY_SIZE(mwifiex_rates),
 };
 
 static struct ieee80211_channel mwifiex_channels_5ghz[] = {