[POWERPC] 4xx: Fix Bamboo MAL0 dts entry.

According to PowerPC 440EP documentation,
MAL0 consists of 6 channels (4 transmit channels and 2 receive channels)
This patch fixes Bamboo DTS MAL0 "num-rx-chans" entry.

Signed-off-by: Valentine Barshak <vbarshak@ru.mvista.com>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>