shell/sc_can: fix uninitialized warning [backport 2018.07]

[cladmi]

Backport of #9627

Contribution description

When compiled for hifive1 board with gcc-7.2.0 this warning was raised:

'ret' may be used uninitialized in this function [-Werror=maybe-uninitialized]

This was first mentioned in RIOT-OS/riotdocker#42 (comment)

Testing

The board does not have enough memory but the error is triggered when compiling with a gcc version 7.2.0:

make -C tests/conn_can/ BOARD=hifive1 RIOT_CI_BUILD=1

Warning is dismissed, it was just a testing procedure error.

Warning

Currently when compiling with riscv-none-embed-gcc (GNU MCU Eclipse RISC-V Embedded GCC, 64-bits) 7.2.0 a linker error is raised

make -C tests/conn_can/ BOARD=hifive1
...
/opt/gnu-mcu-eclipse/riscv-none-gcc/7.2.0-2-20180111-2230/bin/../lib/gcc/riscv-none-embed/7.2.0/../../../../riscv-none-embed/bin/ld: section .stack VMA [0000000080003c00,0000000080003fff] overlaps section .bss VMA [00000000800001f8,0000000080003eaf]
collect2: error: ld returned 1 exit status
/data/riotbuild/riotbase/Makefile.include:356: recipe for target '/data/riotbuild/riotproject/tests/conn_can/bin/hifive1/tests_conn_can.elf' failed
make: *** [/data/riotbuild/riotproject/tests/conn_can/bin/hifive1/tests_conn_can.elf] Error 1
/home/user/Desktop/RIOT_master_old/makefiles/docker.inc.mk:90: recipe for target '..in-docker-container' failed
make: *** [..in-docker-container] Error 2

The goal of this PR is to resolve the linking bug and merge this.

Issues/PRs references

#9405
RIOT-OS/riotdocker#42
Found it during release testing

[miri64]

Ok, change looks simple and I confirmed, that in the current release branch this issue exist and it is indeed fixed by this PR.