net: phy: Add a helper to return the index for of the internal delay

Add a helper function that will return the index in the array for the
passed in internal delay value.  The helper requires the array, size and
delay value.

The helper will then return the index for the exact match or return the
index for the index to the closest smaller value.

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>