rtl8180: avoid NULL dereference in max2820_rf_set_channel

The static function max2820_rf_set_channel is called with conf == NULL
within its compilation unit.  Originally this defaulted to b/g channel
1, but "cfg80211 API for channels/bitrates, mac80211 and driver
conversion" (commit 8318d78a) mistakenly
dropped this check.  This patch minimally restores the expected
behavior.

Reported-by: Colin Lai <colin_sh@163.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>