staging: rtl8192u: ieee80211: Fix incorrect type in assignment

Added le16_to_cpu() conversion fixing the following warning in assignment detected by sparse:

drivers/staging/rtl8192u/ieee80211/ieee80211_softmac.c:2157:30: warning: invalid assignment: +=
drivers/staging/rtl8192u/ieee80211/ieee80211_softmac.c:2157:30:    left side has type unsigned long
drivers/staging/rtl8192u/ieee80211/ieee80211_softmac.c:2157:30:    right side has type restricted __le16

Signed-off-by: Gaston Gonzalez <gascoar@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>