mips: fix erros on registers macros of pll-ddr-config1-nfrac for QCA956X

See details in chapter 8.6.2 and 8.6.4 (page 140-141) of qca9563 datasheet,

   NFRAC[17:0]

So the mask of [17:5] is 0x1fff not 0x3fff.

Signed-off-by: Rosy Song <rosysong@rosinson.com>

Changes for v2-v3:
   - add more information for this commit

Changes for v4-v5:
   - coding style cleanup
1 file changed