dell_rbu: use scnprintf() instead of less secure sprintf()

commit 81156928f8fe31621e467490b9d441c0285998c3 upstream.

Reading 0 bytes from /sys/devices/platform/dell_rbu/image_type or
/sys/devices/platform/dell_rbu/packet_size by an ordinary user causes an
oops.

Signed-off-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>