rtw88: coex: fix missing unitialization of variable 'interval'

Currently the variable 'interval' is not initialized and is only set
to 1 when oex_stat->bt_418_hid_existi is true.  Fix this by inintializing
variable interval to 0 (which I'm assuming is the intended default).

Addresses-Coverity: ("Uninitalized scalar variable")
Fixes: 5b2e9a35 ("rtw88: coex: add feature to enhance HID coexistence performance")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201203175142.1071738-1-colin.king@canonical.com