USB: gadget: ffs: Fix USBCV compliance failure due to adbd

Userspace f_fs client e.g. ADB daemon can re-open ep0 file node
on adb_read/write error (or on bus reset). This results in device
disabling and enabling pull-up on bus reset. USBCV CH9 tests which
perform bus resets also fail due to this.
Fix this by not failing adb_read on bus reset so that adbd client
woudln't attempt ep0-file close. This behavior is not changed for
composition switch or cable disconnect by checking if adb_read
was attempting first transfer after online state or it was in middle
of a session when disconnect happened.

Change-Id: I2378f22995b7ddffb17d618df059d8aedb594d97
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>