platform/x86/amd/pmf: Fix clang unused variable warning

platform/x86/amd/pmf: Fix clang unused variable warning

variable 'mode' is used uninitialized whenever switch default is taken
in sps.c which leads to the following clang warning.

drivers/platform/x86/amd/pmf/sps.c:96:2: error: variable 'mode' is used uninitialized whenever switch default is taken [-Werror,-Wsometimes-uninitialized]
          default:
          ^~~~~~~
  drivers/platform/x86/amd/pmf/sps.c:101:9: note: uninitialized use occurs here
          return mode;
                 ^~~~
  drivers/platform/x86/amd/pmf/sps.c:84:9: note: initialize the variable 'mode' to silence this warning
          u8 mode;
                 ^
                  = '\0'
  1 error generated.

Fix it by returning -EOPNOTSUPP in default case and also change the return
type of the function amd_pmf_get_pprof_modes() to keep it similar like
other drivers which implement platform_profile.

Fixes: 4c71ae414474 ("platform/x86/amd/pmf: Add support SPS PMF feature")
Reported-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Shyam Sundar S K <Shyam-sundar.S-k@amd.com>
Link: https://lore.kernel.org/r/20220822062917.4061503-1-Shyam-sundar.S-k@amd.com
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>