rtlwifi: rtl818x: silence uninitialized variable warning

What about if "rtlphy->pwrgroup_cnt" is 2?  In that case we would use an
uninitialized "chnlgroup" variable and probably crash.  Maybe that can't
happen for some reason which is not obvious but in that case this patch
is harmless.

Setting it to zero seems like a standard default in the surrounding code
so it's probably fine here as well.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>