mailbox: mpfs: ditch a useless busy check

mpfs_mbox_rx_data() already checks if the system controller is busy
before attempting to do anything, so drop the second check before
reading any data.

No functional change intended.

Acked-by: Jassi Brar <jaswinder.singh@linaro.org>
Tested-by: Valentina Fernandez <valentina.fernandezalanis@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>