systemc: Don't use write() in sc_clock to set the clock value.

The two different processes we have to use to match Accellera behavior
would trip the checks in write.

Change-Id: Iee729b239bccd8f7b854894000c9ef2c47600563
Reviewed-on: https://gem5-review.googlesource.com/c/13202
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
1 file changed