mmc: am654_sdhci: Use sdhci_set_control_reg()

Use the generic sdhci_set_control_reg() instead of duplicating
in platform driver.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>