MIPS: BCM63XX: fix ENETDMA_6345_MAXBURST_REG offset

MIPS: BCM63XX: fix ENETDMA_6345_MAXBURST_REG offset

The channels are only 0x40 bytes large, so 0x40 would be the next one's
CHANCFG_REG. Also the position makes it clear that this was intended to
be 0x04. So clearly a typo.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: bcm-kernel-feedback-list@broadcom.com
Patchwork: https://patchwork.linux-mips.org/patch/15316/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>