bsps/shared/dev/serial/*.c: Correct comparisons of different signedness

The GCC warning -Wsign-compare flagged these instances of comparing
signed and unsigned types. A common pair was size_t and int.

bsps/shared/dev/serial/mc68681.c

@@ -212,7 +212,7 @@ MC68681_STATIC void mc68681_initialize_context(
   mc68681_context  *pmc68681Context
 )
 {
-  int          port;
+  unsigned long port;
   unsigned int pMC68681;
   unsigned int pMC68681_port;
 
@@ -432,7 +432,7 @@ MC68681_STATIC rtems_isr mc68681_isr(
   rtems_vector_number vector
 )
 {
-  int     minor;
+  unsigned long minor;
 
   for(minor=0 ; minor<Console_Port_Count ; minor++) {
     if(Console_Port_Tbl[minor]->ulIntVector == vector &&
@@ -517,7 +517,7 @@ MC68681_STATIC ssize_t mc68681_write_support_polled(
   size_t      len
 )
 {
-  int nwrite = 0;
+  size_t nwrite = 0;
 
   /*
    * poll each byte in the string out of the port.

bsps/shared/dev/serial/ns16550.c

@@ -850,7 +850,7 @@ ssize_t ns16550_write_support_polled(
   size_t      len
 )
 {
-  int nwrite = 0;
+  size_t nwrite = 0;
 
   /*
    * poll each byte in the string out of the port.

bsps/shared/dev/serial/z85c30.c

@@ -662,7 +662,7 @@ Z85C30_STATIC rtems_isr z85c30_isr(
   rtems_vector_number vector
 )
 {
-  int                 minor;
+  unsigned long       minor;
   uint32_t            ulCtrlPort;
   volatile uint8_t    ucIntPend;
   volatile uint8_t    ucIntPendPort;
@@ -847,7 +847,7 @@ Z85C30_STATIC ssize_t z85c30_write_support_polled(
   const char *buf,
   size_t len)
 {
-  int nwrite=0;
+  size_t nwrite=0;
 
   /*
    * poll each byte in the string out of the port.