net: sfp: rename T_PROBE_WAIT to T_SERIAL

SFF-8472 rev 12.2 defines the time for the serial bus to become ready
using t_serial.  Use this as our identifier for this timeout to make
it clear what we are referring to.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>