bandlimited: fix compilation

bandlimited.c:145:30: warning: dereferencing ‘void *’ pointer
bandlimited.c:145:30: error: request for member ‘i_buf_size’ in something not a structure or union
bandlimited.c:303:46: error: invalid application of ‘sizeof’ to incomplete type ‘struct filter_sys_t’
bandlimited.c:333:26: warning: dereferencing ‘void *’ pointer
bandlimited.c:333:26: error: request for member ‘p_buf’ in something not a structure or union