staging: rtl8192e: Add _rtl92e_wait_for_fw()

Add function waiting for firmware load/boot;
Remove _rtl92e_is_fw_ready() that is obsolete (can be replaced
with  _rtl92e_wait_for_fw().

Signed-off-by: Mateusz Kulikowski <mateusz.kulikowski@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>