tty: serial: bcm63xx_uart: remove unused inclusion

bcm63xx_irqs.h is included but we are not using anything from it, drop
that include.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6205/