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>