pwm: hibvt: Constify hibvt_pwm_ops

File size before:
   text	   data	    bss	    dec	    hex	filename
   1510	    296	      0	   1806	    70e	drivers/pwm/pwm-hibvt.o
File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   1606	    192	      0	   1798	    706	drivers/pwm/pwm-hibvt.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>