tee: tee_shm: Constify dma_buf_ops structures.

tee: tee_shm: Constify dma_buf_ops structures.

dma_buf_ops are not supposed to change at runtime. All functions
working with dma_buf_ops provided by <linux/dma-buf.h> work with
const dma_buf_ops. So mark the non-const structs as const.

File size before:
   text	   data	    bss	    dec	    hex	filename
   2026	    112	      0	   2138	    85a	drivers/tee/tee_shm.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   2138	      0	      0	   2138	    85a	drivers/tee/tee_shm.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>