rtlwifi: sparse warnings: cast to restricted type

Adding type casts to suppress sparse warnings:
 * warning: cast to restricted __le32/__le16

Signed-off-by: Mark Schulte <schultetwin@gmail.com>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>