staging: gasket: Check the return value of gasket_get_bar_index()

Check the return value of gasket_get_bar_index function as it can return
a negative one (-EINVAL). If this happens, a negative index is used in
the "gasket_dev->bar_data" array.

Addresses-Coverity-ID: 1438542 ("Negative array index read")
Fixes: 9a69f508

 ("drivers/staging: Gasket driver framework + Apex driver")
Signed-off-by: Oscar Carter <oscar.carter@gmx.com>
Cc: stable <stable@vger.kernel.org>
Reviewed-by: Richard Yeh <rcy@google.com>
Link: https://lore.kernel.org/r/20200501155118.13380-1-oscar.carter@gmx.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>