[aacraid] Add missing unmap call for SYNC mode

This issue was observed on a PowerPC64 machine with an Adaptec RAID Controller
with PCI device ID 0x028d. After several read/write operations, the kernel was
panic'ing in bus_dmamap_sync(). This was due to a missing aac_unmap_command()
in the SYNC path.

PR: 237463
Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D23668