mmc: atmel-mci: change atmci_start_command to atmci_send_command

Rename atmci_start_command() to atmci_send_command() which is more
appropriate; atmci_start_command suggests we're sending a start command.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Chris Ball <cjb@laptop.org>