powerpc/mpic: Pass first free vector number to mpic_setup_error_int()

Update the comment to account for the spurious interrupt number. The
code was already accounting for it, but that was unclear because it
was achieved by mpic_setup_error_int() knowing that the number it was
passed was the last used vector, rather than the first free vector.

So change the meaning of the argument to the first free vector and
update the caller to pass 13, instead of 12, to achieve the same
result.
Signed-off-by: Bharat Bhushan <Bharat.Bhushan@nxp.com>
[mpe: Rewrite change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/sysdev/fsl_mpic_err.c

@@ -76,7 +76,7 @@ int mpic_setup_error_int(struct mpic *mpic, int intvec)
 	mpic->flags |= MPIC_FSL_HAS_EIMR;
 	/* allocate interrupt vectors for error interrupts */
 	for (i = MPIC_MAX_ERR - 1; i >= 0; i--)
-		mpic->err_int_vecs[i] = --intvec;
+		mpic->err_int_vecs[i] = intvec--;
 
 	return 0;
 }

arch/powerpc/sysdev/mpic.c

@@ -1380,12 +1380,12 @@ struct mpic * __init mpic_alloc(struct device_node *node,
 		 * global vector number space, as in case of ipis
 		 * and timer interrupts.
 		 *
-		 * Available vector space = intvec_top - 12, where 12
+		 * Available vector space = intvec_top - 13, where 13
 		 * is the number of vectors which have been consumed by
-		 * ipis and timer interrupts.
+		 * ipis, timer interrupts and spurious.
 		 */
 		if (fsl_version >= 0x401) {
-			ret = mpic_setup_error_int(mpic, intvec_top - 12);
+			ret = mpic_setup_error_int(mpic, intvec_top - 13);
 			if (ret)
 				return NULL;
 		}