wil6210: fix chan check in wil_p2p_listen

In wil_p2p_listen chan is checked to protect against NULL
pointer access only before setting channel = chan->hw_value.
Add a global parameter check to cover all accesses to chan.

Signed-off-by: Maya Erez <qca_merez@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>