lc15: Fix returning values on void function
lc15: Fix returning values on void function

Fixes following warnings:
/osmo-bts/src/osmo-bts-litecell15/l1_if.c:1722:36: warning: assignment to ‘void (*)(void *)’ from incompatible pointer type ‘int (*)(void *)’ [-Wincompatible-pointer-types]
 1722 |  fl1h->hw_alive.dsp_alive_timer.cb = dsp_alive_timer_cb;
      |                                    ^

Change-Id: I7a944c0636933c0389db9aabd8b5e6c173052f12

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts