ufshci: reserve one queue entry for an admin request

This patch reduces num_entries by 1 to reserve a admin slot. It
resolves the issue where admin requests cannot be executed
during QD32 because all queue entries are used.

It also resolve the issue of accessing the queue before it is
created by moving the queue construct point to before interrupt
enable.

Sponsored by: Samsung Electronic
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D51894