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 <[e-mail]>
Cc: Christian König <[e-mail]>
Cc: Alex Deucher <[e-mail]>
Signed-off-by: Srinivasan Shanmugam <[e-mail]>
Reviewed-by: Lijo Lazar <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/amdgpu/imu_v12_0.c

···
 static int imu_v12_0_init_microcode(struct amdgpu_device *adev)
 {
 	char fw_name[40];
-	char ucode_prefix[30];
+	char ucode_prefix[15];
 	int err;
 	const struct imu_firmware_header_v1_0 *imu_hdr;
 	struct amdgpu_firmware_info *info = NULL;