Fix a build error caused by not including <linux/bug.h>.

The following compilation errors are caused by the missing header:

arch/mips/ralink/mt7620.c: In function ‘mt7620_get_cpu_pll_rate’:
arch/mips/ralink/mt7620.c:431:2: error: implicit declaration of function ‘WARN_ON’ [-Werror=implicit-function-declaration]
  WARN_ON(div >= ARRAY_SIZE(mt7620_clk_divider));
  ^
arch/mips/ralink/mt7620.c: In function ‘mt7620_get_sys_rate’:
arch/mips/ralink/mt7620.c:500:2: error: implicit declaration of function ‘WARN’ [-Werror=implicit-function-declaration]
  if (WARN(!div, "invalid divider for OCP ratio %u", ocp_ratio))
  ^
arch/mips/ralink/mt7620.c: In function ‘mt7620_dram_init’:
arch/mips/ralink/mt7620.c:619:3: error: implicit declaration of function ‘BUG’ [-Werror=implicit-function-declaration]
   BUG();
   ^
cc1: some warnings being treated as errors
scripts/Makefile.build:302: recipe for target 'arch/mips/ralink/mt7620.o' failed
Signed-off-by: Harvey Hunt <harvey.hunt@imgtec.com>
Cc: John Crispin <john@phrozen.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/16781/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>