drivers: ethernet: bugfix for Cyclone V Ethernet

bugfix for Cyclone V Ethernet Phy error and timeout overflow.
 - p->instance was incorrectly assumed to be a reference to
   the emac device, this is ammended
 - the volatile uint16_t timeout would often overflow
 - code cleanup and added more macros for housekeeping

Signed-off-by: Benjamin Kyd <benjamin.kyd@intel.com>