mtd: spi-nor: stm32-quadspi: Fix uninitialized error return code

With gcc 4.1.2:

    drivers/mtd/spi-nor/stm32-quadspi.c: In function ‘stm32_qspi_tx_poll’:
    drivers/mtd/spi-nor/stm32-quadspi.c:230: warning: ‘ret’ may be used uninitialized in this function

Indeed, if stm32_qspi_cmd.len is zero, ret will be uninitialized.
This length is passed from outside the driver using the
spi_nor.{read,write}{,_reg}() callbacks.

Several functions in drivers/mtd/spi-nor/spi-nor.c (e.g. write_enable(),
write_disable(), and erase_chip()) call spi_nor.write_reg() with a zero
length.

Fix this by returning an explicit zero on success.

Fixes: 0d43d7ab ("mtd: spi-nor: add driver for STM32 quad spi flash controller")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Ludovic Barre <ludovic.barre@st.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@wedev4u.fr>