drivers: net: tulip: Fix set but not used with W=1

drivers: net: tulip: Fix set but not used with W=1

When compiled for platforms other than __i386__ or __x86_64__:

drivers/net/ethernet/dec/tulip/tulip_core.c: In function ‘tulip_init_one’:
drivers/net/ethernet/dec/tulip/tulip_core.c:1296:13: warning: variable ‘last_irq’ set but not used [-Wunused-but-set-variable]
 1296 |  static int last_irq;

Add more #if defined() to totally remove the code when not needed.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20201031005445.1060112-1-andrew@lunn.ch
Signed-off-by: Jakub Kicinski <kuba@kernel.org>