ti: common: board_detect: Replace hardcoded value with macro

We should have used TI_DEAD_EEPROM_MAGIC in the first place.

Fixes: d3b98a9e ("ti: common: dra7: Add standard access for board description EEPROM")
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>