Input: synaptics-rmi4 - use dev_driver_string when registering interrupt

When IRQ handling was moved to rmi_driver in 3aeed5b5 the naming of the
interrupt changed from "rmi4_i2c" to "2-0020" (or similar). This patch
restores the previous behaviour and makes the interrupt easier to identify
in /proc/interrupts.

Signed-off-by: Nick Dyer <nick@shmanahar.org>
Tested-by: Chris Healy <cphealy@gmail.com>
Acked-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>