usb: gadget: f_fs: stop sleeping in ffs_func_eps_disable

ffs_func_eps_disable is called from atomic context so it cannot sleep
thus cannot grab a mutex.  Change the handling of epfile->read_buffer
to use non-sleeping synchronisation method.

Reported-by: Chen Yu <chenyu56@huawei.com>
Signed-off-by: Michał Nazarewicz <mina86@mina86.com>
Fixes: 9353afbb

 ("buffer data from ‘oversized’ OUT requests")
Tested-by: John Stultz <john.stultz@linaro.org>
Tested-by: Chen Yu <chenyu56@huawei.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>