gpu: nova-core: gsp: fix incorrect advancing of write pointer

gpu: nova-core: gsp: fix incorrect advancing of write pointer

We should modulo not bitwise-and here. The current code could, for
example, set wptr to MSGQ_NUM_PAGES which is not valid.

Fixes: 75f6b1de8133 ("gpu: nova-core: gsp: Add GSP command queue bindings and handling")
Signed-off-by: Eliot Courtney <ecourtney@nvidia.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260129-nova-core-cmdq1-v3-1-2ede85493a27@nvidia.com
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>