usb: musb: gadget: kill unreachable code in musb_g_rx()

musb_g_rx() always returns if next_request() call yields NULL, so the DBG()
near the function's end can never be invoked. Remove it along with unneeded
'return'; also remove the duplicate 'request' check...

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>