ena: Update default PHC expiration timeout

PHC expiration timeout (expire_timeout_usec) represent the max time
for valid PHC retrieval, passing this threshold will fail the get time
request and will block new PHC requests for block_timeout_usec in order
to prevent floods on busy device.

PHC expiration timeout is set by the device or by default driver value.
This commit updates the default driver expiration timeout from 20 to 10
(usec) to decrease the overall error bound committed to the user.

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