staging/dgap: Use strpbrk() instead of dgap_sindex()

The <linux/string.h> provides strpbrk() function that does
the same that the dgap_sindex(). Let's use already defined
function instead of writing custom.

Signed-off-by: Alexander Kuleshov <kuleshovmail@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>