fix(w5100): wait 83ms after software reset, not 1ms

The init does MR=0x80 (software reset) then waits reset_cycles before writing
NETLCKR/SHAR/SIPR/socket registers.  That wait was 24_000 cycles = 1 ms at
24 MHz, but the W5100S needs the datasheet's 60.3 ms -- our own bus self-test
needed ~85 ms before the chip answered at all.

1 ms is marginal rather than simply wrong, which is why this presented as
intermittent: an NCRA reset sometimes left the chip still resetting while the
init writes landed on it and were silently dropped, leaving it misconfigured
and the bus master blocked.  Symptom was a W5100 that wedged after an NCRA
reset -- TX never completing and the UART shell's own 'udp unicast' hanging --
recoverable only by a 12 V power cycle.  Note RST_N is never asserted by the
gateware; only the software reset is used, so this wait is the only settle
time the chip gets.

Default is now 2_000_000 = 83 ms.  Testbenches pass small values explicitly and
are unaffected.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

exi_bba/bba_top.py

@@ -48,7 +48,7 @@ class BBATop(Elaboratable):
         w5500_rst_n                            : W5500 hardware reset (output, active low)
     """
 
-    def __init__(self, eth="w5100", reset_cycles=24000,
+    def __init__(self, eth="w5100", reset_cycles=2_000_000,
                  status_panel=False, uart_console=False, uart_shell=False,
                  udp_socket=3, udp_src_ip="192.168.1.123",
                  udp_subnet="255.255.255.0", udp_gateway="192.168.1.1",