USB: g_printer, fix empty if statement

A bug every C programmer makes at some point in time...

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Craig W. Nadler <craig@nadler.us>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>