watchdog: gef_wdt: fix MODULE_ALIAS

Remove the space between "platform:" prefix and the driver name.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Martyn Welch <martyn.welch@ge.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>