nouveau/gsp/r535: remove a stray unlock in r535_gsp_rpc_send()

This unlock doesn't belong here and it leads to a double unlock in
the caller, r535_gsp_rpc_push().

Fixes: 176fdcbd ("drm/nouveau/gsp/r535: add support for booting GSP-RM")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Timur Tabi <ttabi@nvidia.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/a0293812-c05d-45f0-a535-3f24fe582c02@moroto.mountain