travis-ci: use correct exit code on errors

The phrase "if [ $? -ne 0 ]; then exit $?; fi" doesn't work correctly;
by the time the "exit" statement runs, $? has already been over-written
by the result of the [ command. Fix this by explicitly storing $? and
then using that stored value in both the test and the error-case exit
statement.

This change also converts from textual comparison to integer comparison,
since the exit code is an integer and there's no need to convert it to
a string for comparison.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Heiko Schocher <hs@denx.de>