accel/amdxdna: Fix incorrect size of ERT_START_NPU commands

When multiple ERT_START_NPU commands are combined in one buffer, the
buffer size calculation is incorrect. Also, the condition to make sure
the buffer size is not beyond 4K is also fixed.

Fixes: aac24309 ("accel/amdxdna: Add command execution")
Reviewed-by: Jacek Lawrynowicz <jacek.lawrynowicz@linux.intel.com>
Reviewed-by: Maciej Falkowski <maciej.falkowski@linux.intel.com>
Signed-off-by: Lizhi Hou <lizhi.hou@amd.com>
Link: https://lore.kernel.org/r/20250409210013.10854-1-lizhi.hou@amd.com