mvebu: limit mvneta tx queue workaround to 32 bit SoC

mvebu: limit mvneta tx queue workaround to 32 bit SoC

This patch has been carried since introduction throughout every kernel
major bump and no one has tested if the later kernels improved the
situation. The Armada 3720 SoC can only process GbE interrupts on Core 0
and this is already limited in all stable kernels, so ditch this
workaround for 64 bit SoCs.

Ref: https://git.kernel.org/torvalds/c/cf9bf871280d

Signed-off-by: Tomasz Maciej Nowak <tmn505@gmail.com>