Fix build for kernel 5.14

[alexmohr]

See the following commits in slcan.c

• https://github.com/torvalds/linux/commit/357a6a875f1c4772f2102639bf19619780889f31#diff-a7fff3312b7439498e6c17c72dcf9434e7c6007952fac08efb418ad4d0743a96
• https://github.com/torvalds/linux/commit/f81ee8b8b8421dc06d13f197bb53191559cc51da#diff-a7fff3312b7439498e6c17c72dcf9434e7c6007952fac08efb418ad4d0743a96
• https://github.com/torvalds/linux/commit/fbadf70a8053b3dce78a45997ae55651693a2a81#diff-a7fff3312b7439498e6c17c72dcf9434e7c6007952fac08efb418ad4d0743a96
• https://github.com/torvalds/linux/commit/0f3dcf3b5d76669123bf99fec812b8b0acd60375#diff-a7fff3312b7439498e6c17c72dcf9434e7c6007952fac08efb418ad4d0743a96