ARM: 8750/1: deflate_xip_data.sh: minor fixes

Send nm complaints about broken pipe (when sed exits early) to /dev/null.
All errors should be printed to stderr.
Don't trap on normal exit so the trap can return an error code.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Tested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>