[WATCHDOG] davinci_wdt clean-up

* Remove the redundant check for pwrite(), given that the open() routine
  already invokes nonseekable_open().
* The WDIOF_CARDRESET flag can only be used when you can read this status
  via the WDIOC_GETSTATUS ioctl call.
* Add the mandatory WDIOC_GETBOOTSTATUS ioctl call.

Signed-off-by: Wim Van Sebroeck <wim@iguana.be>