wifi: atmel: Avoid clashing function prototypes

When built with Control Flow Integrity, function prototypes between
caller and function declaration must match. These mismatches are visible
at compile time with the new -Wcast-function-type-strict in Clang[1].

Of the 1549 warnings found, 188 come from the atmel driver. For example:

drivers/net/wireless/atmel/atmel.c:2518:2: warning: cast from 'int (*)(struct net_device *, struct iw_request_info *, void *, char *)' to 'iw_handler' (aka 'int (*)(struct net_device *, struct iw_request_info *, union iwreq_data *, char *)') converts to incompatible function type [-Wcast-function-type-strict]
        (iw_handler) atmel_config_commit,       /* SIOCSIWCOMMIT */
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The atmel Wireless Extension handler callbacks (iw_handler) use a union
for the data argument. Actually use the union and perform explicit
member selection in the function body instead of having a function
prototype mismatch. There are no resulting binary differences.

This patch is a cleanup based on Brad Spengler/PaX Team's modifications
to the atmel driver in their last public patch of grsecurity/PaX based
on my understanding of the code. Changes or omissions from the original
code are mine and don't reflect the original grsecurity/PaX code.

[1] https://reviews.llvm.org/D134831

Cc: Simon Kelley <[email protected]>
Cc: Kalle Valo <[email protected]>
Cc: "David S. Miller" <[email protected]>
Cc: Eric Dumazet <[email protected]>
Cc: Jakub Kicinski <[email protected]>
Cc: Paolo Abeni <[email protected]>
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Kees Cook <[email protected]>
Signed-off-by: Kalle Valo <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: kees