net: ethernet: Add helper functions to update error stats.

Add missed out helper functions to update the errors.tx and error.rx
in the stats structure.

Signed-off-by: Fadhel Habeeb <fadhel@zilogic.com>
Signed-off-by: Nirav Parmar <niravparmar@zilogic.com>
Signed-off-by: Vijay Kumar B <vijaykumar@zilogic.com>