usb: dwc3: core: introduce and use macros for Event Size register

That register has more than just the event buffer
size; we can also mask and unmask that particular
interrupter on bit 31 of that register.

In this patch we introduce the necessary macros
and make sure to use the new macros while also
making sure we mask interrupts during driver
removal.

Signed-off-by: Felipe Balbi <balbi@ti.com>