ena: Set ena_min_poll_delay_us default value

This commit sets the default value for ena_min_poll_delay_us to 100.

This commit does not change the behavior of the driver, the delay is
calculated as MAX(ENA_MIN_ADMIN_POLL_US, delay_us), where the first
field is already defined as 100.
The second parameter, delay_us is taken from ena_min_poll_delay_us
which is currently unset - 0.

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