spi: stm32_qspi: correct static analysis issues

spi: stm32_qspi: correct static analysis issues

Sparse issue:
drivers/st/spi/stm32_qspi.c:445:5:
 warning: symbol 'stm32_qspi_init' was not declared. Should it be static?

Cppcheck issue:
[drivers/st/spi/stm32_qspi.c:175] -> [drivers/st/spi/stm32_qspi.c:187]:
 (style) Variable 'len' is reassigned a value before the old one has been
 used.
[drivers/st/spi/stm32_qspi.c:178]:
 (style) The scope of the variable 'timeout' can be reduced.

Change-Id: I575fb50766355a6717cbd193fc4a80ff1923014c
Signed-off-by: Yann Gautier <yann.gautier@st.com>