mmc: mtk-sd: add WATCHDOG_RESET() to prevent watchdog timeout

When reading large data in once (reading 512MiB is tested on MT7623), a
watchdog timeout is triggered due to watchdog not being fed.

This patch adds WATCHDOG_RESET() to msdc_start_data() so the watchdog will
be fed every 1024 blocks are read/written.

Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>