USB: serial: garmin_gps: fixes package loss if used from gpsbabel

This patch contains two fixes submitted by Ondrej Palkovsky:
- the 'ACK' packet is sent after the transfer of the USB packet is
completed, i.e. in the write_callback function. Because the close
function sends the 'abort' command, a parameter is added that allows
the caller of garmin_write_bulk to specify, if the 'ack' should be
propagated to the serial link or dimissed.
This fixes the problem with gpsbabel, it has sent several packets that
were acknowledged before they were sent to the GPS and GpsBabel closed
the device - thus effectively cancelled all outstanding requests in the
queue.
- removed the APP_RESP_SEEN and APP_REQ_SEEN flags and changed
them into counters. It evades USB reset of the gps on every device close.

Signed-off-by: Hermann Kneissel <hermann.kneissel@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>