Staging: rtl8192e: mark symbols static where possible

We get a few warnings when building kernel with W=1:
drivers/staging/rtl8192e/rtllib_softmac.c:279:13: warning: no previous declaration for 'softmac_ps_mgmt_xmit' [-Wmissing-declarations]
drivers/staging/rtl8192e/rtllib_softmac.c:773:24: warning: no previous declaration for 'rtllib_authentication_req' [-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.

In addition, some of these functions are declared in different files,
it looks like that we need to clean the codes of this driver up,
but we can repress these warnings first, then clean it up.

so this patch marks these functions with 'static' now.

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