staging: r8188eu: replace rtw_netdev_priv define with inline function

The function guarantees type checking of arguments and return value.

Result of rtw_netdev_priv macro can be assigned to pointer
with incompatible type without warning. The function allow compiler
to perform this check.

Signed-off-by: Ivan Safonov <insafonov@gmail.com>
Link: https://lore.kernel.org/r/20200801180235.34116-1-insafonov@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>