[rtl] Fix typo in MCOUNTEREN
[rtl] Fix typo in MCOUNTEREN

DbgTriggerEn has no impact on whether this is an illegal csr op.

Fixes #1379

Signed-off-by: Tom Roberts <f3b1a53ee1cbeb35a47033dea932030093853292@lowrisc.org>

lowRISC/ibex,lowRISC/ibex,AmbiML/ibex,AmbiML/ibex,lowRISC/ibex,AmbiML/ibex,AmbiML/ibex,lowRISC/ibex