staging: r8188eu: cleanups in rtw_android_set_block()

1) We can tighten up the code a little by returning directly and it
   makes the code more future proof and easier to read.
2) "free" is a better name than "exit".
3) sizeof(priv_cmd) is shorter and more clear than sizeof(struct
   android_wifi_priv_cmd).

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>