flash/stmqspi: fix build error with -Werror=maybe-uninitialized

flash/stmqspi: fix build error with -Werror=maybe-uninitialized

using gcc 9.3 on ubuntu focal fossa with -Werror=maybe-uninitialized
we get this error:
  /src/flash/nor/stmqspi.c: In function ‘read_flash_id’:
  /src/flash/nor/stmqspi.c:1948:6: error: ‘retval’ may be used uninitialized

Change-Id: Ifd8ae60df847fc61e22ca100c008e3914c9af79b
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@st.com>
Reviewed-on: http://openocd.zylin.com/6012
Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>