hostapd: fix "bad number" error due to missing wps_pbc_in_m1 option (since r39995)

r39995 introduced a new parameter wps_pbc_in_m1 to wifi wps config, but
apparently did not provide a default value 0.

When that option's non-existing value is later evaluated in
/lib/netifd/hostapd.sh, it causes the "bad number" error to be logged in
syslog if user has not set the wps_pbc_in_m1 option. The error materialises
only if user has enabled wps.
    Sat Apr 12 13:25:01 2014 daemon.notice netifd: radio1 (1254): sh: bad number
    Sat Apr 12 13:25:01 2014 daemon.notice netifd: radio0 (1253): sh: bad number

Discussion in bug 15508: https://dev.openwrt.org/ticket/15508#comment:3

Error is caused by line 282:
https://dev.openwrt.org/browser/trunk/package/network/services/hostapd/files/netifd.sh#L282

My patch sets the parameter's default value to 0, which does nothing. The
default might also be set a bit later in the function, but this felt like the
most clear place to do that.

Signed-off-by hnyman <hannu.nyman@iki.fi>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40469 3c298f89-4303-0410-b956-a3cf2f4a3e73