mmc: mediatek: Preinitialize delay_phase in get_best_delay()

drivers/mmc/host/mtk-sd.c: In function ‘get_best_delay’:
drivers/mmc/host/mtk-sd.c:1284: warning: ‘delay_phase.start’ is used uninitialized in this function
drivers/mmc/host/mtk-sd.c:1284: warning: ‘delay_phase.maxlen’ is used uninitialized in this function

If delay is zero, these fields are indeed not initialized.
Let the compiler preinitialize the whole struct to fix this.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>