drivers: flash_mspi_nor: Remove undesirable initial Quad disabling

Using a GPIO reset for a flash chip that has a dual function pin
(RESET# or SIO3) and is to be used in Quad mode is rather a bad idea
and so is clearing of the Quad Enable bit at every initialization
of the flash driver, since this bit is usually non-volatile, so such
operation means unnecessary wearing of the flash chip. Soft Reset
should be use instead in such case.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>