classes/kernel.bbclass: Fix parsing errors

legitimize_package_name wants the actual value of KERNEL_REVISION, so
use d.getVar() to fetch it as is done elsewhere in the file. Failing to
do so can result it weird errors at parsing time.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>