net: phy: at803x: make array offsets static

Don't populate the read-only const array offsets on the stack
but instead make it static. Also makes the object code a little smaller.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20220109231716.59012-1-colin.i.king@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>