Fix detection of IBM Power8 machines (ISA 2.07)

An IBM POWER7 system with Power ISA 2.06 tried to execute
zfs_sha256_power8() - which should only be run on ISA 2.07
machines.

The detection is implemented via the zfs_isa207_available() call,
but this check was not used.

This pull request will fix this.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: Low-power <msl0000023508@gmail.com>
Closes #14576