MIPS: Loongson64: Remove a "set but not used" variable

This fix build warning:

   arch/mips/loongson64/env.c: In function 'prom_init_env':
>> arch/mips/loongson64/env.c:50:14: warning: variable 'device' set but not used [-Wunused-but-set-variable]
      50 |  u16 vendor, device;
         |              ^~~~~~
   {standard input}: Assembler messages:
   {standard input}:788: Error: found '(', expected: ')'
   {standard input}:788: Error: found '(', expected: ')'
   {standard input}:788: Error: non-constant expression in ".if" statement
   {standard input}:788: Error: junk at end of line, first unrecognized character is `('
   {standard input}:801: Error: found '(', expected: ')'
   {standard input}:801: Error: found '(', expected: ')'
   {standard input}:801: Error: non-constant expression in ".if" statement
   {standard input}:801: Error: junk at end of line, first unrecognized character is `('
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/loongson64/env.c

@@ -62,7 +62,7 @@ void __init prom_lefi_init_env(void)
 	struct efi_cpuinfo_loongson *ecpu;
 	struct irq_source_routing_table *eirq_source;
 	u32 id;
-	u16 vendor, device;
+	u16 vendor;
 
 	/* firmware arguments are initialized in head.S */
 	boot_p = (struct boot_params *)fw_arg2;
@@ -166,7 +166,6 @@ void __init prom_lefi_init_env(void)
 	/* Read the ID of PCI host bridge to detect bridge type */
 	id = readl(HOST_BRIDGE_CONFIG_ADDR);
 	vendor = id & 0xffff;
-	device = (id >> 16) & 0xffff;
 
 	switch (vendor) {
 	case PCI_VENDOR_ID_LOONGSON: