net: mvneta: Rename CONFIG_NR_CPUS to MVNETA_NR_CPUS

The CONFIG_* macros are reserved for Kconfig. This was probably done
when this driver was being imported from Linux. Rename the macro.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>