drivers/interrupt_controller: Fix command write in VT-D

A surprising issue: while sys_set_bit() on VTD_GCMD_REG work on
apollo_lake, it seems to create border effect on elkhart_lake: it may
reset some previous bit to 0.

So switching to full write of the register at once, which works for
both.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>