mem: Reverse order of write/read mem queue check

mem: Reverse order of write/read mem queue check

For atomic RMW instructions that go directly to memory, we want to put
them on the write queue instead of the read queue. Swap the if/else
condition to accomplish this.

Note: This is ignoring the read latency of the RMW, but these
instructions should usually be handled in caches anyway.

Change-Id: I62dbfff3a16ac470f1ebdb489abe878962b20bb6
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17828
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>