drivers: wifi: esp_at: Fix serial receive interrupt can't exit

If a character is received immediately after modem_iface_uart_init called,
the modem_iface_uart_isr function will not read the data in the serial port
fifo register as the context has not been registered at this time,
which will result in the program not being able to exit the interrupt,
so the context should registered before the serial port is initialised.

Signed-off-by: Hongquan Li <hongquan.prog@gmail.com>