i2c: fix i2c_dw.c to have correct decl for i2c_dw_isr()

The function i2c_dw_isr() in this file appears to have the wrong
decl for the IRQ_CONNECT macro. It should be: void i2c_dw_isr(void *arg).
Also, at the 2nd use of IRQ_CONNECT there was a variable created
and then marked as UNUSED. I've removed this.

Change-Id: I3f2c4ecb5c13147d577c82a0ec4fe486ae38a475
Signed-off-by: Chuck Jordan <cjordan@synopsys.com>