Fix zfs-dkms .deb package warning in prerm script

Debian zfs-dkms package generated by alien doesn't call the prerm script
(rpm's %preun) with an integer as first parameter, which results in the
following warning when the package is uninstalled:

   "zfs-dkms.prerm: line 3: [: remove: integer expression expected"

Modify the if-condition to avoid the warning.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #9271