powerpc/64e: Fix system call illegal mtmsrd instruction

BookE does not have mtmsrd, switch to use wrteei to enable MSR[EE].

Fixes: dd152f70

 ("powerpc/64s: system call avoid setting MSR[RI] until we set MSR[EE]")
Reported-by: Christian Zigotzky <chzigotzky@xenosoft.de>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210706051310.608992-1-npiggin@gmail.com