drivers: flash: restore stm32 xspi mutex on memory mapping

Fix stm32 XSPI driver to restore the bus command concurrent access
protection that was mistakenly removed on memory mapping operation
by commit e5620e07 ("drivers: flash: stm32 xspi flash read with
memcopy when executing").

Signed-off-by: Etienne Carriere <etienne.carriere@st.com>