bsps/sparc: Address -Wsign-compare warnings

This warning occurs when comparing a signed variable to an unsigned one.
This addresses warnings that only occurred on 64-bit targets.  For the
ones which only appeared on 64-bit targets, the cause was frequently
a mismatch when comparing a combination off_t, ssize_t, and int.

bsps/sparc/erc32/console/erc32_console.c

@@ -292,7 +292,7 @@ static ssize_t erc32_console_write_support_polled(
   size_t      len
 )
 {
-  int nwrite = 0;
+  size_t nwrite = 0;
 
   while (nwrite < len) {
     console_outbyte_polled( minor, *buf++ );

bsps/sparc/leon2/console/console.c

@@ -283,7 +283,7 @@ void console_outbyte_interrupt(
 
 static ssize_t console_write_support (int minor, const char *buf, size_t len)
 {
-  int nwrite = 0;
+  size_t nwrite = 0;
 
   while (nwrite < len) {
 #if (CONSOLE_USE_INTERRUPTS)