drivers: serial: fix console hang when waken up from PM3

The console can't input when waken up from PM3, as get the wrong
usart_intenset when process PM_DEVICE_ACTION_TURN_ON and the
USART interrupt is not enabled. Only saving usart_intenset after
PM action of PM_DEVICE_ACTION_TURN_OFF can fix this issue.

Signed-off-by: Maochen Wang <maochen.wang@nxp.com>