mailbox: bcm-flexrm-mailbox: Use txdone_ack instead of txdone_poll

Currently, FlexRM driver uses txdone_poll method of Linux Mailbox
to model the send_data() callback. To achieve this, we have introduced
"last_pending_msg" pointer for each FlexRM ring which keeps track of
the message that did not fit in the FlexRM ring.

This patch updates FlexRM driver to use txdone_ack method instead of
txdone_poll method because txdone_poll is not efficient for FlexRM
and requires additional tracking in FlexRM driver.

Also, moving to txdone_ack method helps us remove "last_pending_msg"
pointer and last_tx_done() callback.

Signed-off-by: Anup Patel <anup.patel@broadcom.com>
Reviewed-by: Ray Jui <ray.jui@broadcom.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>