drivers: mmc: iproc_sdhci: fix compilation warning

set_ios_post return type changed from void to int, correcting
the same to fix compilation warning.

Signed-off-by: Rayagonda Kokatanur <rayagonda.kokatanur@broadcom.com>
Signed-off-by: Bharat Kumar Reddy Gooty <bharat.gooty@broadcom.com>