staging: rtl8712: Replace r8712_free_cmd_priv()

Remove function r8712_free_cmd_priv as all it does is call
_free_cmd_priv.
Change type of new r8712_free_cmd_priv from static to non-static to
match definition of original r8712_free_cmd_priv.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>