usb: gadgetfs: return number of bytes on ep0 read request

commit 85b4b3c8c189e0159101f7628a71411af072ff69 upstream.

A read from GadgetFS endpoint 0 during the data stage of a control
request would always return 0 on success (as returned by
wait_event_interruptible) despite having written data into the user
buffer.
This patch makes it correctly set the return value to the number of
bytes read.

Signed-off-by: Thomas Faber <thfabba@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>