broadcom-wl: fix compilation with kernel 5.4

broadcom-wl: fix compilation with kernel 5.4

This adds two fixes for compilation with kernel 5.4:

1. dev_open from include/linux/netdevice.h needs a second parameter
   since kernel 5.0:
   00f54e68924e ("net: core: dev: Add extack argument to dev_open()")
2. get_ds() macro definition has been dropped since kernel 5.1:
   736706bee329 ("get rid of legacy 'get_ds()' function")
   Since get_ds() has been just a macro before, replace it in
   the driver instead of creating a version switch.

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