Staging: rtl8192u: use gfp_t instead of int

This patch fixes the following type of sparse warnings:

drivers/staging/rtl8192u/ieee80211/ieee80211_tx.c:247:17: warning: incorrect type in argument 2 (different base types)
drivers/staging/rtl8192u/ieee80211/ieee80211_tx.c:247:17:    expected restricted gfp_t [usertype] flags
drivers/staging/rtl8192u/ieee80211/ieee80211_tx.c:247:17:    got int [signed] gfp_mask

Signed-off-by: Teodora Baluta <teobaluta@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>