Bug 1639096: Fixing setting of $limit in all submissions ajax

The existing code was slightly wrong when compared to the other ajax
files in this dir.

Though probably wouldn't have caused an issue as the the ajax wouldn't
have been called if the block didn't allow submissions - so this fix
is to just tidy up the code.

behatnotneeded

Change-Id: Ia715b53379bde6abe43591c2ca36623be1b07b90
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>