staging: dgap: driver.c: removes smatch warning "redundant null check"

This patch removes these smatch warnings from dgap_driver.c:

redundant null check on dgap_config_buf calling kfree()
redundant null check on brd->flipbuf calling kfree()
redundant null check on brd->flipflagbuf calling kfree()

Signed-off-by: Lidza Louina <lidza.louina@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>