usb: musb: core: decrease delayed_work time

When babble IRQ happens, we need to wait only
5.3us (320 cycles of 60MHz clock), we will give
it some slack and schedule our work a 10 usecs into
the future.

Tested-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>