usb: musb: core: fix TX/RX endpoint order

As per Mentor Graphics' documentation, we should
always handle TX endpoints before RX endpoints.

This patch fixes that error while also updating
some hard-to-read comments which were scattered
around musb_interrupt().

This patch should be backported as far back as
possible since this error has been in the driver
since it's conception.

Cc: <stable@vger.kernel.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>