usb: musb: core: refactor IRQ enable/disable to separate functions

sometimes we want to just mask/unmask interrupts
without touching devctl register. For those
cases, let's introduce musb_enable_interrupts and
musb_disable_interrupts()

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