arch: arm: userspace: correct inline comment for bad syscalls

We need to correct the inline comment in swap_helper.S,
which is suggesting that system call attempts with
invalid syscall IDs (i.e. above the limit) do not force
the CPU to elevate privileges. This is in fact not true,
since the execution flow moves into valid syscall ID
handling.

In other words, all we do for system calls with invalid
ID numbers is to treat them as valid syscalls with the
K_SYSCALL_BAD ID value.

We fix the inline documentation to reflect the actual
execution flow.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>