drivers: mailbox: rename mbox_send_controller_data

Rename mbox_send_controller_data to mbox_write_controller_data, since
the data is never actually sent, but only written to the controller.

Change-Id: I450802ea6e7dc870c85ab8d5473985c7882105e6
Signed-off-by: Lina Iyer <ilina@codeaurora.org>