staging/rtl8192u: add endianness conversions

Fields frag_size and playload_size of struct ieee80211_txb are encoded
as short little-endian. This patch adds conversions to / from cpu byte
order when copy / write these values in variables of architecture
independent byte order. It also avoid a sparse type warning.

Signed-off-by: Colin Vidal <colin@cvidal.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>