mailbox: ti-msgmgr: Refactor message read during interrupt handler

Refactor the portion of code that actually reads received messages from
a queue into its own function, ti_msgmgr_queue_rx_data, that is called
by the interrupt handler instead of reading directly from the handler.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>