Ignore requests to switch to RATP mode in input-only or output-only
consoles.

This actually also avoids segfaulting later on:

    #0  0x0000000000000000 in ?? ()
    #1  0x000000000040b2a4 in console_send (r=<optimized out>, pkt=0x7fffffffd8ec, len=4) at common/ratp.c:102
    #2  0x000000000042ab99 in ratp_behaviour_a (pkt=0x7ffff7298780, ri=0x7ffff72766d0) at lib/ratp.c:530
    #3  ratp_state_machine (pkt=0x7ffff7298780, ri=0x7ffff72766d0) at lib/ratp.c:1384
    #4  ratp_poll (ratp=0x7ffff7277ed8) at lib/ratp.c:1561
    #5  0x000000000042b2ab in ratp_establish (ratp=ratp@entry=0x7ffff7277ed8, active=active@entry=false, timeout_ms=timeout_ms@entry=100) at lib/ratp.c:1645
    #6  0x000000000040b888 in barebox_ratp (cdev=cdev@entry=0x7ffff7212bd0) at common/ratp.c:470
    #7  0x00000000004046c3 in getc_raw () at common/console.c:416
    ...

Signed-off-by: Aleksander Morgado <aleksander@aleksander.es>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>