systemc: Fix accounting in the sc_fifo class.

Reads shouldn't free up space until an update happens.

Change-Id: I18e1601c27b44643f103c86f04b1fa2c23baf1e8
Reviewed-on: https://gem5-review.googlesource.com/c/13333
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
1 file changed