stk: Fix sizeof on memcpy

stk: Fix sizeof on memcpy

src/stk.c: In function ‘__ofono_cbs_sim_download’:
src/stk.c:283:45: error: argument to ‘sizeof’ in ‘memcpy’ call is the
same expression as the source; did you mean to dereference it?
[-Werror=sizeof-pointer-memaccess]
  memcpy(&e.cbs_pp_download.page, msg, sizeof(msg));
                                               ^