staging: dgap: remove redundant error value check

The retval in dgap_block_til_ready() is initialized to zero,
and if no error has occurred in this function, the retval has a zero.
So it doesn't need to check "retval" itself.

Signed-off-by: Daeseok Youn <daeseok.youn@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>