staging: rtl8712: rename bDriverStopped to driver_stopped

 - Rename structure field bDriverStopped to driver_stopped to avoid
camelcase. Suggested by checkpatch.
 - Change type to bool as the variable is being used as boolean.

Signed-off-by: Himadri Pandya <himadri18.07@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>