GAS: Clear offchannel_tx_started when ending remain-on-channel

GAS: Clear offchannel_tx_started when ending remain-on-channel

Commit 2c0d0ae370f72fbe6248feed7ea0635303e3dc5a ('GAS: End
remain-on-channel due to delayed GAS comeback request') started ending
the remain-on-channel operation between the initial request and the
following comeback request. However, it did not check or update the
offchannel_tx_started variable. While this alone would not necessarily
be problematic, this makes it more difficult to optimize wait time for
offchannel TX operations, so make sure the internal tracking variable
gets updated.

Signed-off-by: Jouni Malinen <j@w1.fi>