staging: gasket: gasket_mmap return error instead of valid BAR index

When offset to be mapped matches both a BAR region and a coherent mapped
region return an error as intended, not the BAR index.

Signed-off-by: Simon Que <sque@chromium.org>
Signed-off-by: Todd Poynor <toddpoynor@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>