mmc: sdhci-bcm-kona: Staticize sdhci_bcm_kona_card_event

sdhci_bcm_kona_card_event is referenced only in this file.
Make it static.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Christian Daudt <csd@broadcom.com>