[POWERPC] fix spin lock nesting in hvc_iseries

We had nested spinlocks using the same flags variable, but it turns out
that we don't need the nested locks at all (the lock protects a static
buffer that we aren't using here), so just remove the extra locks.

Spotted by Alexey Dobriyan.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>