rtw88: 8822c: fix missing brace warning for old compilers

For older versions of gcc, the array = {0}; will cause warnings:

drivers/net/wireless/realtek/rtw88/rtw8822c.c: In function 'rtw8822c_power_trim':
>> drivers/net/wireless/realtek/rtw88/rtw8822c.c:1039:2: warning:
>> missing braces around initializer [-Wmissing-braces]
s8 bb_gain[2][8] = {0};
^
drivers/net/wireless/realtek/rtw88/rtw8822c.c:1039:2: warning: (near
initialization for 'bb_gain[0]') [-Wmissing-braces]

Fixes: 5ad4d895 ("rtw88: set power trim according to efuse PG values")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200522035521.12295-1-yhchuang@realtek.com