powerpc64: Avoid calling isync twice

[breno.leitao_gmail.com]

Summary

It is not necessary to call isync() after calling mtmsr() function, mainly
because the mtmsr() calls 'isync' internally to synchronize the machine state
register. Other than that, isync() just calls the 'isync' instruction, thus,
the 'isync' instruction is being called twice, and that seems to be unnecessary.

This patch just remove the unecessary calls to isync() after mtmsr().