qcacld-3.0: Use variable size structure to store channel in scan req

Currently the array of scan channel list is of size
SIR_ESE_MAX_MEAS_IE_REQS, but the memory is allocated dynamically
for the channge which can be greater than SIR_ESE_MAX_MEAS_IE_REQS.

So use dynamic array for this as memory is allocated for this every
time.

Change-Id: I3c854b339c49d9f628033aa6742d57568ec14954
CRs-Fixed: 2560184