powerpc/sysdev: Fix comment typo

The double `is' is duplicated in line 110, remove one.

Signed-off-by: Jason Wang <wangborong@cdjrlc.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220715035250.5978-1-wangborong@cdjrlc.com