net: mdio: mdio-bcm-unimac: Delay before first poll

With a clock interval of 400 nsec and a 64 bit transactions (32 bit
preamble & 16 bit control & 16 bit data), it is reasonable to assume
the mdio transaction will take 25.6 usec. Add a 30 usec delay before
the first poll to reduce the chance of a 1000-2000 usec sleep.

Reduce the timeout from 1000ms to 100ms as it is unlikely for the bus
to take this long.

Signed-off-by: Justin Chen <justin.chen@broadcom.com>
Acked-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20231213222744.2891184-2-justin.chen@broadcom.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>