drm/amdgpu/swsmu: fix ARC build errors

We want to use the dev_* functions here rather than the pr_* variants.
Switch to using dev_warn() which mirrors what we do on other asics.

Fixes the following build errors on ARC:

../drivers/gpu/drm/amd/amdgpu/../powerplay/navi10_ppt.c: In function 'navi10_fill_i2c_req':
../arch/arc/include/asm/bug.h:24:2: error: implicit declaration of function 'pr_warn'; did you mean 'drm_warn'? [-Werror=implicit-function-declaration]

../drivers/gpu/drm/amd/amdgpu/../powerplay/sienna_cichlid_ppt.c: In function 'sienna_cichlid_fill_i2c_req':
../arch/arc/include/asm/bug.h:24:2: error: implicit declaration of function 'pr_warn'; did you mean 'drm_warn'? [-Werror=implicit-function-declaration]

Reported-by: kernel test robot <lkp@intel.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Evan Quan <evan.quan@amd.com>
Cc: Vineet Gupta <vgupta@synopsys.com>
Cc: linux-snps-arc@lists.infradead.org
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>