ravb: Add missing free_irq() call to ravb_close()

When reopening the network device on ra7795/salvator-x, e.g. after a
DHCP timeout:

    IP-Config: Reopening network devices...
    genirq: Flags mismatch irq 139. 00000000 (eth0:ch24:emac) vs. 00000000 (eth0:ch24:emac)
    ravb e6800000.ethernet eth0: cannot request IRQ eth0:ch24:emac
    IP-Config: Failed to open eth0
    IP-Config: No network devices available

The "mismatch" is due to requesting an IRQ that is already in use,
while IRQF_PROBE_SHARED wasn't set.

However, the real cause is that ravb_close() doesn't release the R-Car
Gen3-specific secondary IRQ.

Add the missing free_irq() call to fix this.

Fixes: 22d4df8f ("ravb: Add support for r8a7795 SoC")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>