net: axienet: increase reset timeout

The previous timeout of 1ms was too short to handle some cases where the
core is reset just after the input clocks were started, which will
be introduced in an upcoming patch. Increase the timeout to 50ms. Also
simplify the reset timeout checking to use read_poll_timeout.

Fixes: 8a3b7a25

 ("drivers/net/ethernet/xilinx: added Xilinx AXI Ethernet driver")
Signed-off-by: Robert Hancock <robert.hancock@calian.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>