drivers: flash: spi_nor: Fix unused function warning

Building with clang warns:

drivers/flash/spi_nor.c:306:20: error: unused function
'delay_until_exit_dpd_ok' [-Werror,-Wunused-function]
static inline void delay_until_exit_dpd_ok(const struct device *const dev)
                   ^

delay_until_exit_dpd_ok is only used when ANY_INST_HAS_DPD is defined.

Signed-off-by: Tom Hughes <tomhughes@chromium.org>