base: Fix CircularQueue's operator-= when negative subtraction

base: Fix CircularQueue's operator-= when negative subtraction

Using operator-= when the rhs is a negative value is equivalent
to using += on -rhs. This is fixing rounding in that scenario.

Change-Id: Ia22e51f81a6805d27fd6b2115d288bb23421d00f
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17528
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>