mvneta: Try to fix mvneta when compiled as module

When the mvneta driver is compiled as module, the clock is disabled before
it's loading. This will reset the registers values and all configuration
made by the bootloader.

This patch sets the "sgmii serdes configuration" register to a magical value
found in:
https://github.com/yellowback/ubuntu-precise-armadaxp/blob/master/arch/arm/mach-armadaxp/armada_xp_family/ctrlEnv/mvCtrlEnvLib.c



With this change, the interrupts are working/generated and ethernet is
working.

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Signed-off-by: David S. Miller <davem@davemloft.net>