USB: musb_gadget: remove pointless loop

Remove the pointless 'do () while (0)' loop from musb_g_tx() -- it
makes this function symmetric to musb_g_rx()...

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