accel/amdxdna: Fix out-of-bounds memset in command slot handling

accel/amdxdna: Fix out-of-bounds memset in command slot handling

The remaining space in a command slot may be smaller than the size of
the command header. Clearing the command header with memset() before
verifying the available slot space can result in an out-of-bounds write
and memory corruption.

Fix this by moving the memset() call after the size validation.

Fixes: 3d32eb7a5ecf ("accel/amdxdna: Fix cu_idx being cleared by memset() during command setup")
Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>
Signed-off-by: Lizhi Hou <lizhi.hou@amd.com>
Link: https://patch.msgid.link/20260217185415.1781908-1-lizhi.hou@amd.com