qcacld-3.0: Make max supported vdevs as 4

Currently, number of vdevs required can be configured through
the ini param gNumVdevs.
Firmware can accommodate maximum 4 vdevs and the ini param
gNumVdevs indicates the same.
If host driver is going to create vdev for NAN, it indicates
the total no.of vdevs supported to firmware which includes the
NAN vdev.
If firmware is going to create NAN discovery vdev, host should
indicate 3 vdevs and firmware shall add 1 vdev for NAN. So decrement
the num_vdevs by 1.

Change-Id: I9a4fc9ac3bf4ce62fa3388643e047a7a1d546f4d
CRs-Fixed: 2649478