xen/pvcalls: change CONFIG_XEN_PVCALLS_FRONTEND to bool

commit  5909f1977f978a2729dfbdf450dc6161d99cda27 from
https://github.com/Xilinx/linux-xlnx.git xlnx_rebase_v5.10

The newly introduced drivers/xen/pvcalls.c doesn't build when
CONFIG_XEN_PVCALLS_FRONTEND=m due to the presence of the early_param
named "pvcalls".

It is not possible to turn "pvcalls" into a module_param because it is
needed at fs_init time (see net/ipv4/af_inet.c:inet_init).

We could fix the build problem by keeping "pvcalls" as early_param and
moving it away from drivers/xen/pvcalls.c. However, it would introduce
another issue: pvcalls=1 triggers the usage of pvcalls_stream_ops as
networking functions. pvcalls_stream_ops relies on pvcalls.c. If
pvcalls.c is not yet loaded but pvcalls_stream_ops is already enabled,
we would run into issues at runtime.

From the analysis it becomes clear that CONFIG_XEN_PVCALLS_FRONTEND
should be a bool instead of a tristate (module not allowed). This patch
makes the change.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
State: pending
Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>