Staging: rtl8723bs: Change return type and type of local variable

rtw_startbss_cmd can return the result of calling rtw_enqueue_cmd, which
can return the result of calling _rtw_enqueue_cmd, but they all have
different return types (u8, u32, and sint, respectively).

As all of these functions return error codes, change all the return types
to int, in preparation for converting these functions to use standard
kernel error codes.

Also, remove extern from the function declaration to maintain Linux kernel
coding style.

Issue suggested by Coccinelle.

Signed-off-by: Madhumitha Prabakaran <madhumithabiw@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>