usb: musb: host: remove duplicate check in musb_ep_program()

musb_ep_program() contains obviously duplicate check for 'dma_channel' in its
IN/receive path -- removing it allows to save one level of indentation. While
at it, improve the comment style...

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>