USB: usbserial - fix potential deadlock between write() and IRQ

usb_serial_generic_write() doesn't disable interrupts when taking port->lock,
and could therefore deadlock with usb_serial_generic_read_bulk_callback()
being called from interrupt, taking the same lock. Fix it.
Signed-off-by: NJiri Kosina <jkosina@suse.cz>
Acked-by: NLarry Finger <larry.finger@lwfinger.net>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>