serial: 8250: Fix PM usage_count for console handover

In the Linux kernel, the following vulnerability has been resolved:

serial: 8250: Fix PM usage_count for console handover

When console is enabled, univ8250_console_setup() calls

serial8250_console_setup() before .dev is set to uart_port. Therefore,

it will not call pm_runtime_get_sync(). Later, when the actual driver

is going to take over univ8250_console_exit() is called. As .dev is

already set, serial8250_console_exit() makes pm_runtime_put_sync() call

with usage count being zero triggering PM usage count warning

(extra debug for univ8250_console_setup(), univ8250_console_exit(), and

serial8250_register_ports()):

[ 0.068987] univ8250_console_setup ttyS0 nodev

[ 0.499670] printk: console [ttyS0] enabled

[ 0.717955] printk: console [ttyS0] printing thread started

[ 1.960163] serial8250_register_ports assigned dev for ttyS0

[ 1.976830] printk: console [ttyS0] disabled

[ 1.976888] printk: console [ttyS0] printing thread stopped

[ 1.977073] univ8250_console_exit ttyS0 usage:0

[ 1.977075] serial8250 serial8250: Runtime PM usage count underflow!

[ 1.977429] dw-apb-uart.6: ttyS0 at MMIO 0x4010006000 (irq = 33, base_baud = 115200) is a 16550A

[ 1.977812] univ8250_console_setup ttyS0 usage:2

[ 1.978167] printk: console [ttyS0] printing thread started

[ 1.978203] printk: console [ttyS0] enabled

To fix the issue, call pm_runtime_get_sync() in

serial8250_register_ports() as soon as .dev is set for an uart_port

if it has console enabled.

This problem became apparent only recently because 82586a721595 ("PM:

runtime: Avoid device usage count underflows") added the warning

printout. I confirmed this problem also occurs with v5.18 (w/o the

warning printout, obviously).