mmc: sdhci: Handle execute tuning command in sdhci_send_command

This patch upadted sdhci_send_command to handle execute tuning
command.

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>