mfd: rtsx: Add clock divider hook

Add callback function conv_clk_and_div_n to convert between SSC clock
and its divider N.
For rtl8411, the formula to calculate SSC clock divider N is different
with the other card reader models.

Signed-off-by: Wei WANG <wei_wang@realsil.com.cn>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
diff --git a/drivers/mfd/rts5209.c b/drivers/mfd/rts5209.c
index ba74de8..98fe0f3 100644
--- a/drivers/mfd/rts5209.c
+++ b/drivers/mfd/rts5209.c
@@ -174,6 +174,7 @@
 	.card_power_off = rts5209_card_power_off,
 	.switch_output_voltage = rts5209_switch_output_voltage,
 	.cd_deglitch = NULL,
+	.conv_clk_and_div_n = NULL,
 };
 
 /* SD Pull Control Enable: