ZTS: Fix mmp_interval failure

The mmp_interval test case was failing on Fedora 30 due to the built-in
'echo' command terminating the script when it was unable to write to
the sysfs module parameter. This change in behavior was observed with
ksh-2020.0.0-alpha1. Resolve the issue by using the external cat
command which fails gracefully as expected.

Additionally, remove some incorrect quotes around the $? return values.

Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #8906