[submodule] Advance sonic-linux-kernel pointer

Update sonic-linux-kernel submodule pointer to include the following:
* 6847319 Read ID register for optoe1 to find pageable bit in optoe driver  ([sonic-net#308](sonic-net/sonic-linux-kernel#308))
* 42ad073 Add markers to faciliate easy parsing of kconfig-inclusion and series ([sonic-net#309](sonic-net/sonic-linux-kernel#309))

Signed-off-by: Vivek Reddy <[email protected]>