handwired/onekey/blackpill_f401: Fix I2C pin config mismatch (#10322)

By default the `i2c_master` driver for ChibiOS uses the B6 pin for
`I2C1_SCL` and the B7 pin for `I2C1_SDA`.  However, the ChibiOS board
file used for the F401 Blackpill board (`ST_STM32F401C_DISCOVERY`)
configures B6 as `I2C1_SCL` and B9 as `I2C1_SDA`, and if that
configuration is left unchanged, enabling the `i2c_master` driver
results in having two pins (B7 and B9) configured as `I2C1_SDA` at the
same time, which does not work properly (experimental results show that
the B9 pin still works as `I2C1_SDA` in that case, and the B7 pin does
not work).

Configure the B9 pin as an input with pull-up in `board_init()`, so that
the B7 pin can be configured as `I2C1_SDA` by the I2C driver.

keyboards/handwired/onekey/blackpill_f401/blackpill_f401.c

@@ -0,0 +1,23 @@
+/* Copyright 2020 Sergey Vlasov (sigprof)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include QMK_KEYBOARD_H
+
+void board_init(void) {
+    // B9 is configured as I2C1_SDA in the board file; that function must be
+    // disabled before using B7 as I2C1_SDA.
+    setPinInputHigh(B9);
+}