input: jdi-bu21150: amend device tree parsing error check

Operations to read optional device tree entries return negative
values when errors occur. These error values are negative and
should be checked against properly. Otherwise, driver will fail
when these entries are not defined.

Change-Id: I58d92ef4fba4d867401c69cd690aabe37b84771f
Signed-off-by: Chun Zhang <chunz@codeaurora.org>