b43: Remove uninitialized_var() usage

Using uninitialized_var() is dangerous as it papers over real bugs[1]
(or can in the future), and suppresses unrelated compiler warnings (e.g.
"unused variable"). If the compiler thinks it is uninitialized, either
simply initialize the variable or make compiler changes. As a precursor
to removing[2] this[3] macro[4], just initialize this variable to NULL.
No later NULL deref is possible due to the early returns outside of the
(phy->rev >= 7 && phy->rev < 19) case, which explicitly tests for NULL.

[1] https://lore.kernel.org/lkml/20200603174714.192027-1-glider@google.com/
[2] https://lore.kernel.org/lkml/CA+55aFw+Vbj0i=1TGqCR5vQkCzWJ0QxK6CernOU6eedsudAixw@mail.gmail.com/
[3] https://lore.kernel.org/lkml/CA+55aFwgbgqhbp1fkxvRKEpzyR5J8n1vKT1VZdz9knmPuXhOeg@mail.gmail.com/
[4] https://lore.kernel.org/lkml/CA+55aFz2500WfbKXAx8s67wrm9=yVJu65TpLgN_ybYNv0VEOKA@mail.gmail.com/

Fixes: 58619b14 ("b43: move under broadcom vendor directory")
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200620033007.1444705-5-keescook@chromium.org

drivers/net/wireless/broadcom/b43/phy_n.c

@@ -4222,7 +4222,7 @@ static void b43_nphy_tx_gain_table_upload(struct b43_wldev *dev)
 	u32 rfpwr_offset;
 	u8 pga_gain, pad_gain;
 	int i;
-	const s16 *uninitialized_var(rf_pwr_offset_table);
+	const s16 *rf_pwr_offset_table = NULL;
 
 	table = b43_nphy_get_tx_gain_table(dev);
 	if (!table)