staging: rtl8192u: ieee80211: ieee80211_softmac: mark symbols static where possible

We get 5 warnings when building kernel with W=1:
drivers/staging/rtl8192u/ieee80211/ieee80211_softmac.c:287:13: warning: no previous declaration for 'softmac_ps_mgmt_xmit' [-Wmissing-declarations]
drivers/staging/rtl8192u/ieee80211/ieee80211_softmac.c:323:24: warning: no previous declaration for 'ieee80211_probe_req' [-Wmissing-declarations]
drivers/staging/rtl8192u/ieee80211/ieee80211_softmac.c:643:24: warning: no previous declaration for 'ieee80211_authentication_req' [-Wmissing-declarations]
drivers/staging/rtl8192u/ieee80211/ieee80211_softmac.c:981:24: warning: no previous declaration for 'ieee80211_association_req' [-Wmissing-declarations]
drivers/staging/rtl8192u/ieee80211/ieee80211_softmac.c:3094:24: warning: no previous declaration for 'ieee80211_disassociate_skb' [-Wmissing-declarations]

In fact, these functions are only used in the file in which they are
declared and don't need a declaration, but can be made static.
so this patch marks these functions with 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>