icE1usb: Use register bit #define instead of magic number

Change-Id: Ic824ce5a6605c96989fefbf92f365bff618dccb2
diff --git a/firmware/ice40-riscv/icE1usb/e1.c b/firmware/ice40-riscv/icE1usb/e1.c
index 776edba..5d4d3ee 100644
--- a/firmware/ice40-riscv/icE1usb/e1.c
+++ b/firmware/ice40-riscv/icE1usb/e1.c
@@ -406,7 +406,7 @@
 		return;
 
 	/* HACK: LED link status */
-	if (e1_regs->rx.csr & 2)
+	if (e1_regs->rx.csr & E1_RX_SR_ALIGNED)
 		led_color(0, 48, 0);
 	else
 		led_color(48, 0, 0);