samples: can: shell send fix

This fixes the limitation of sending an extended CAN frame.
Because the default value of `CONFIG_SHELL_ARGC_MAX` is 12 the maximum
number of bytes for an extended CAN frame is 7.

Increasing it to 13 to allow all 8 bytes to be sent from the shell.

Tested on nucleo_f767zi.

Signed-off-by: Cezar Burlacu <cezar@embeddedp.ro>