usb: musb: core: improve musb_interrupt() a bit

instead of using manually spelled out bit-shits
and iterate over each of the 16-bits (one for
each endpoint) on each direction, we can make use
of for_each_set_bit() which internally uses
find_first_bit().

This makes the code slightly more readable while
also making we only iterate over bits which are
actually set.

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