staging: wlan-ng: convert endianness in situ for prism2sta

Fix several sparse warnings about casts to restricted
little-endian by using in situ conversions.

Signed-off-by: Thibaut SAUTEREAU <thibaut.sautereau@telecom-sudparis.eu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>