cpu/lpc2387: fix unused parameter warning

mark 'mode' as unused in pwm_init
Merge pull request #1405 from LudwigOrtmann/pwm_fixup

cpu/lpc2387: fix unused parameter warning
sys/shell: fix error when not using sixlowpan

```
/sys/shell/commands/sc_net_if.c:631:27: error: ‘IPV6_MAX_ADDR_STR_LEN’ undeclared (first use in this function)
             char addr_str[IPV6_MAX_ADDR_STR_LEN];
                           ^
```