staging: rtl8187se: use gfp_t instead of int

Fix the following type of sparse warnings:

drivers/staging/rtl8187se/ieee80211/ieee80211_tx.c:249:17: warning: incorrect type in argument 2 (different base types)
drivers/staging/rtl8187se/ieee80211/ieee80211_tx.c:249:17:    expected restricted gfp_t [usertype] flags
drivers/staging/rtl8187se/ieee80211/ieee80211_tx.c:249:17:    got int [signed] gfp_mask

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