powerpc64: Avoid calling isync twice

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().

Submitted by: Breno Leitao
Differential Revision: https://reviews.freebsd.org/D14583