drivers: espi: xec: Avoid flash interrupt by ROM bootloader activity

When MEC15xx boots in MAFS, ROM bootloader performs activity over
eSPI flash channel prior to FW is loaded.
Need to clear STS_DONE during early eSPI driver initialization,
before enabling interrupts to avoid unexpected ISRs in FW.

Otherwise this would cause flash_lock semaphore to be incremented
due to ROM activity and result in FW's first espi flash request
to return immediately even before eSPI bus transaction completes.

Signed-off-by: Jose Alberto Meza <jose.a.meza.arellano@intel.com>