enc28j60: Fixes an issue with concurrent tx and rx

The ENC28J60 hardware module does not support concurrent
transmission and reception.
This fix adds a control semaphore to exclude the execution
of both processes.

Applies the equivalent changes to the legacy driver.

Jira: ZEP-1097

Change-Id: I9602195d5a97f8d4bf652753c284d61f192357fe
Signed-off-by: Juan Manuel Cruz <juan.m.cruz.alcaraz@intel.com>