selftests: firmware: send expected errors to /dev/null

Error that we expect should not be spilled to stdout.

Without this we get:

./fw_filesystem.sh: line 58: printf: write error: Invalid argument
./fw_filesystem.sh: line 63: printf: write error: No such device
./fw_filesystem.sh: line 69: echo: write error: No such file or directory
./fw_filesystem.sh: filesystem loading works
./fw_filesystem.sh: async filesystem loading works

With it:

./fw_filesystem.sh: filesystem loading works
./fw_filesystem.sh: async filesystem loading works

Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>