rndis_wlan: fix le16/le32_to_cpu mix up with config.beacon_period

'beacon_period' in 'struct ndis_80211_conf' is __le32 instead of __le16 so
le32_to_cpu must be used instead of le16_to_cpu.

Also correct 'beacon_interval' variables used for passing this value forward
from u16 to u32 and rename those variables 'beacon_period' This is to avoid
confusion because 'beacon_interval' is defined as __le16 at other structure,
'struct ndis_80211_fixed_ies'.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>