wifi: zd1201: 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].

Fix a total of 30 warnings like these:

../drivers/net/wireless/zydas/zd1201.c:1560:2: warning: cast from 'int (*)(struct net_device *, struct iw_request_info *, struct iw_freq *, 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) zd1201_set_freq,           /* SIOCSIWFREQ */
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~

The zd1201 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
before/after changes.

These changes were made partly manually and partly with the help of
Coccinelle.

Link: https://github.com/KSPP/linux/issues/233
Link: https://reviews.llvm.org/D134831 [1]
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/5b7fbb1a22d5bfaa872263ca20297de9b431d1ec.1667934775.git.gustavoars@kernel.org