serial: tegra-tcu: Reorder channel initialization

Request the RX mailbox only after initializing the UART data
structures. Otherwise it can rarely happen that the receive
callback is called before the UART is ready.

Fixes: 2d908b38

 ("serial: Add Tegra Combined UART driver")
Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Link: https://lore.kernel.org/r/20210609155655.3567545-1-mperttunen@nvidia.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>