i2c-stm32f4: remove redundant initialization of pointer reg

The pointer reg is assigned a value that is never read, it is later
overwritten with a new value, hence the redundant initialization can
be removed.

Cleans up clang warning:
drivers/i2c/busses/i2c-stm32f4.c:352:16: warning: Value stored to 'reg'
during its initialization is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Alexandre TORGUE <alexandre.torgue@st.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>