staging: gdm724x: modify icmp6_checksum for returning a correct data type.

The icmp6_checksum was returning an invalid data type as the expected type
is __sum16. For returning such data type, icmp6_checksum, now, is using
the kernel functions for computing the checksum.

Here, the sparse message:

drivers/staging/gdm724x/gdm_lte.c:311:39: warning: incorrect type in assignment (different base types)
drivers/staging/gdm724x/gdm_lte.c:311:39:    expected restricted __sum16 [addressable] [assigned] [usertype] icmp6_cksum
drivers/staging/gdm724x/gdm_lte.c:311:39:    got int

Signed-off-by: Javier Rodriguez <jrodbar@yahoo.es>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>