Refactor hard-reset implementation in ahci(4).

Instead of spinning in a tight loop for up to 15 seconds, polling for device
readiness while it spins up, return reset completion just after PHY reports
"connect well" or 100ms connection timeout. If device was found, use callout
for checking device readiness with 100ms period up to full 31 second timeout.

This fixes system freeze for 5-10 seconds on drives hot plug-in.