Make the INT_N line configurable

It needs to be if I want to be able to have several ports controlled by
one MCU.  It's set as part of the FUSB configuration, not in the INT_N
struct, since the latter isn't user-initialized and may disappear
entirely in the future.

lib/include/pdb_fusb.h

     I2CDriver *i2cp;
     /* The I2C address of the chip */
     i2caddr_t addr;
+    /* The INT_N line */
+    ioline_t int_n;
 };
 
 /*

lib/src/int_n.c

 
     while (true) {
         /* If the INT_N line is low */
-        if (palReadLine(LINE_INT_N) == PAL_LOW) {
+        if (palReadLine(cfg->fusb.int_n) == PAL_LOW) {
             /* Read the FUSB302B status and interrupt registers */
             fusb_get_status(&cfg->fusb, &status);
 

src/main.c

 static struct pdb_config pdb_config = {
     .fusb = {
         &I2CD2,
-        FUSB302B_ADDR
+        FUSB302B_ADDR,
+        LINE_INT_N
     },
     .dpm = {
         pdbs_dpm_evaluate_capability,