drm/amdgpu: Fix truncation by resizing ucode_prefix in imu_v12_0_init_microcode

drm/amdgpu: Fix truncation by resizing ucode_prefix in imu_v12_0_init_microcode

This commit fixes potential truncation when writing the string _imu.bin
into the fw_name buffer in the imu_v12_0_init_microcode function in the
imu_v12_0.c file

The ucode_prefix size was reduced from 30 to 15 to ensure the snprintf
function does not exceed the size of the fw_name buffer.

Thus fixing the below with gcc W=1:
drivers/gpu/drm/amd/amdgpu/imu_v12_0.c: In function ‘imu_v12_0_init_microcode’:
drivers/gpu/drm/amd/amdgpu/imu_v12_0.c:51:54: warning: ‘_imu.bin’ directive output may be truncated writing 8 bytes into a region of size between 4 and 33 [-Wformat-truncation=]
   51 |         snprintf(fw_name, sizeof(fw_name), "amdgpu/%s_imu.bin", ucode_prefix);
      |                                                      ^~~~~~~~
drivers/gpu/drm/amd/amdgpu/imu_v12_0.c:51:9: note: ‘snprintf’ output between 16 and 45 bytes into a destination of size 40
   51 |         snprintf(fw_name, sizeof(fw_name), "amdgpu/%s_imu.bin", ucode_prefix);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Cc: Lijo Lazar <lijo.lazar@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>