broadcom-wl: fix compilation with kernel >= 4.15

broadcom-wl: fix compilation with kernel >= 4.15

Since kernel 4.15, init_timer is not available anymore, and has been
replaced by timer_setup. The fixes compilation of wl_linuc.c, which
returned the following errors beforehand (line-wrapped manually):

.../broadcom-wl-5.10.56.27.3/driver/wl_linux.c: In function 'wl_init_timer':
.../broadcom-wl-5.10.56.27.3/driver/wl_linux.c:2576:2: error: implicit
	declaration of function 'init_timer'; did you mean 'init_timers'?
	[-Werror=implicit-function-declaration]
  init_timer(&t->timer);
  ^~~~~~~~~~
  init_timers
.../broadcom-wl-5.10.56.27.3/driver/wl_linux.c:2577:10: error:
	'struct timer_list' has no member named 'data'
  t->timer.data = (ulong) t;
          ^
.../broadcom-wl-5.10.56.27.3/driver/wl_linux.c:2578:20: error: assignment
	to 'void (*)(struct timer_list *)' from incompatible pointer type
	'void (*)(ulong)' {aka 'void (*)(long unsigned int)'}
	[-Werror=incompatible-pointer-types]
  t->timer.function = wl_timer;

This should fix build of several devices on bcm63xx with testing
kernel (4.19).

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>