drivers: ieee802154: Remove unused function

Building with clang warns:

drivers/ieee802154/ieee802154_dw1000.c:277:24: error: unused function
'dwt_reg_read_u16' [-Werror,-Wunused-function]
static inline uint16_t dwt_reg_read_u16(const struct device *dev,
                       ^

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