arch: x86: Fix early_serial build error when using fixed MMIO address

Fix the following complilation error that happens when specifying a
fixed MMIO address for the UART through X86_SOC_EARLY_SERIAL_MMIO8_ADDR:

arch/x86/core/early_serial.c:30:26: error: #if with no expression
   30 | #if DEVICE_MMIO_IS_IN_RAM

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>