drm/msm: Fix speed-bin support not to access outside valid memory

When running the latest kernel on an sc7180 with KASAN I got this
splat:
  BUG: KASAN: slab-out-of-bounds in a6xx_gpu_init+0x618/0x644
  Read of size 4 at addr ffffff8088f36100 by task kworker/7:1/58
  CPU: 7 PID: 58 Comm: kworker/7:1 Not tainted 5.11.0+ #3
  Hardware name: Google Lazor (rev1 - 2) with LTE (DT)
  Workqueue: events deferred_probe_work_func
  Call trace:
   dump_backtrace+0x0/0x3a8
   show_stack+0x24/0x30
   dump_stack+0x174/0x1e0
   print_address_description+0x70/0x2e4
   kasan_report+0x178/0x1bc
   __asan_report_load4_noabort+0x44/0x50
   a6xx_gpu_init+0x618/0x644
   adreno_bind+0x26c/0x438

This is because the speed bin is defined like this:
  gpu_speed_bin: gpu_speed_bin@1d2 {
    reg = <0x1d2 0x2>;
    bits = <5 8>;
  };

As you can see the "length" is 2 bytes. That means that the nvmem
subsystem allocates only 2 bytes. The GPU code, however, was casting
the pointer allocated by nvmem to a (u32 *) and dereferencing. That's
not so good.

Let's fix this to just use the nvmem_cell_read_u16() accessor function
which simplifies things and also gets rid of the splat.

Let's also put an explicit conversion from little endian in place just
to make things clear. The nvmem subsystem today is assuming little
endian and this makes it clear. Specifically, the way the above sc7180
cell is interpreted:

NVMEM:
 +--------+--------+--------+--------+--------+
 | ...... | 0x1d3  | 0x1d2  | ...... | 0x000  |
 +--------+--------+--------+--------+--------+
              ^       ^
             msb     lsb

You can see that the least significant data is at the lower address
which is little endian.

NOTE: someone who is truly paying attention might wonder about me
picking the "u16" version of this accessor instead of the "u8" (since
the value is 8 bits big) or the u32 version (just for fun). At the
moment you need to pick the accessor that exactly matches the length
the cell was specified as in the device tree. Hopefully future
patches to the nvmem subsystem will fix this.

Fixes: fe7952c6 ("drm/msm: Add speed-bin support to a618 gpu")
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>