This fixes the following compile error seen on the mpc85xx target:
  CC [M]  /linux-mpc85xx_p2020/backports-5.7-rc3-1/drivers/net/wireless/intersil/orinoco/main.o
In file included from /builder/shared-workdir/build/staging_dir/toolchain-powerpc_8540_gcc-8.4.0_musl/include/stddef.h:17,
                 from /linux-mpc85xx_p2020/backports-5.7-rc3-1/include/uapi/linux/wireless.h:77,
                 from /linux-mpc85xx_p2020/backports-5.7-rc3-1/include/linux/wireless.h:13,
                 from /linux-mpc85xx_p2020/backports-5.7-rc3-1/drivers/net/wireless/intersil/orinoco/main.c:89:
/builder/shared-workdir/build/staging_dir/toolchain-powerpc_8540_gcc-8.4.0_musl/include/bits/alltypes.h:106:15: error: conflicting types for 'ptrdiff_t'
 typedef _Addr ptrdiff_t;
               ^~~~~~~~~
In file included from /linux-mpc85xx_p2020/backports-5.7-rc3-1/backport-include/linux/types.h:4,
                 from ./include/linux/list.h:5,
                 from /linux-mpc85xx_p2020/backports-5.7-rc3-1/backport-include/linux/list.h:3,
                 from ./include/linux/module.h:9,
                 from /linux-mpc85xx_p2020/backports-5.7-rc3-1/backport-include/linux/module.h:3,
                 from /linux-mpc85xx_p2020/backports-5.7-rc3-1/drivers/net/wireless/intersil/orinoco/main.c:79:
./include/linux/types.h:65:28: note: previous declaration of 'ptrdiff_t' was here
 typedef __kernel_ptrdiff_t ptrdiff_t;
                            ^~~~~~~~~
scripts/Makefile.build:265: recipe for target '/linux-mpc85xx_p2020/backports-5.7-rc3-1/drivers/net/wireless/intersil/orinoco/main.o' failed

Fixes: 289c6324 ("mac80211: Update to version 5.7-rc3-1")
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>