prima: Dereferencing uninitialized pointer while disabling channels

Accessing n_channels variable from wiphy bands that is
uninitialized which causes NULL pointer dereference in both
wlan_hdd_disable_channels() and wlan_hdd_restore_channels()
functions.

To address this, add NULL check for wiphy bands in both functions.

Change-Id: I937c3d70a6b93768fbe9e2d931dbde60aedc449d
CRs-Fixed: 2320898