mlxbf_gige: increase MDIO polling rate to 5us

This patch increases the polling rate used by the
mlxbf_gige driver on the MDIO bus.  The previous
polling rate was every 100us, and the new rate is
every 5us.  With this change the amount of time
spent waiting for the MDIO BUSY signal to de-assert
drops from ~100us to ~27us for each operation.

Signed-off-by: David Thompson <davthompson@nvidia.com>
Signed-off-by: Asmaa Mnebhi <asmaa@nvidia.com>
Link: https://lore.kernel.org/r/20220505162309.20050-1-davthompson@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>