rtw89: Remove an unneeded __DECONST

Originally in e2340276fc73 the function argument was a
"const void *data" which we changed to "const int" in FreeBSD. The
Linux version simply cast the const void * to (enum rtw89_rf_path)
both losing the const and changing the type. When later the function
argument became a const union rtw89_fw_element_arg in 5b760eaecd6c3 it
seems we tried to undo the older FreeBSD specific part and initially
mismerged it leading to the const void *; but when fixing the type we
did not undo the __DECONST but did a mechanical copy.

Reviewed by: bz
Co-authored-by: bz (commit message)
Obtained from: CheriBSD
Sponsored by: AFRL, DARPA
Fixes: 6d67aabd6355 ("rtw89: update Realtek's rtw89 driver.")
Differential Revision: https://reviews.freebsd.org/D52359