adm5120: avoid warning caused by wrong AMBA irq values

The amba core throws a warning if -1 is used as an
irq value since upstream commit:

  2eac58d5026e4ec8b17ff8b62877fea9e1d2f1b3
  ARM: amba: make use of -1 IRQs warn

  Make the core warn about the use of -1 (NO_IRQ)

Cange the value of the unused IRQs to zero for the
UART devices to get rid of the warning.

Based on #12729.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

SVN-Revision: 35747