srm: Fix set max ready get command

A regression in 2.9.0 caused the 'set max ready get' command of the
SRM to break. The regression is fixed by this patch.

Target: trunk
Require-notes: yes
Require-book: no
Request: 2.10
Request: 2.9
Acked-by: Dmitry Litvintsev <litvinse@fnal.gov>
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7228/
(cherry picked from commit 3c9e3fb8ada286592ae505cbcd43860c9b3b0ce9)