ena: Adjust PHC for multiple devices

Static variables are tied to the driver code and not to each driver
instance, thus, all driver instances, regardless of the devices they
manage, share the same static variables.

This modification switches the static variable "start_time" to be used
within phc struct.

In addition, rearranged several PHC variables for cache friendly
struct layout and refined several variable names and types for
enhanced clarity and accuracy.

Approved by: cperciva (mentor)
MFC after: 2 weeks
Sponsored by: Amazon, Inc.