staging: wlan-ng: prism2mib.c: Fix type cast issues

Fix the following sparse warnings:

prism2mib.c:717:45: warning: cast to restricted __le16
prism2mib.c:720:45: warning: incorrect type in assignment (different base types)
prism2mib.c:720:45:    expected unsigned short [unsigned] [addressable] [usertype] datalen
prism2mib.c:720:45:    got restricted __le16 [usertype] <noident>
prism2mib.c:755:22: warning: incorrect type in assignment (different base types)
prism2mib.c:755:22:    expected unsigned short [unsigned] [usertype] len
prism2mib.c:755:22:    got restricted __le16 [usertype] <noident>

Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>