ath11k: Use sizeof_field() instead of FIELD_SIZEOF()

The FIELD_SIZEOF() macro was redundant, and is being removed from the
kernel. Since commit c593642c ("treewide: Use sizeof_field() macro")
this is one of the last users of the old macro, so replace it.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>