[rstmgr] Fix width mismatch in rstmgr_por.sv
[rstmgr] Fix width mismatch in rstmgr_por.sv

We can safely slice StretchCount here because CtrWidth is defined as
$clog2(StretchCount + 1).

Signed-off-by: Rupert Swarbrick <290093e2ad2515ed2f14d749293c8cf8e02ca274@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan