powerpc/64s: micro-optimise __hard_irq_enable() for mtmsrd L=1 support

Book3S minimum supported ISA version now requires mtmsrd L=1. This
instruction does not require bits other than RI and EE to be supplied,
so __hard_irq_enable() and __hard_irq_disable() does not have to read
the kernel_msr from paca.

Interrupt entry code already relies on L=1 support.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>