staging: gdm724x: Update logging

Make the logging prefixes match the module names
by adding #define pr_fmt(fmt) KBUILD_MODNAME and
converting printks to netdev_<level> when a
struct netdevice * is available and and pr_<level>
when not.

Remove embedded prefixes.
Add missing terminating newlines.
Remove an unnecessary rx-dropped message.
Remove an unnecessary temporary variable.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>