[submodule] Advance sonic-linux-kernel pointer

Update sonic-linux-kernel submodule pointer to include the following:
* 6f38dca Provision the ability to apply the non upstream patches in any order ([sonic-net#313](sonic-net/sonic-linux-kernel#313))
* fbd307e Fix unable to probe emc2301/2/3 ([sonic-net#312](sonic-net/sonic-linux-kernel#312))

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