davinci_nand, trivial : use symbolic ECC start command

The ECC calculations were started by writing 1 << 13 to the nand FCR register;
that value is also defined as DAVINCI_NANDFCR_4BIT_CALC_START in emif_defs.h.

This patch substitutes the macro DAVINCI_NANDFCR_4BIT_CALC_START for the
magic number '1 << 13'.

Signed-off-by: Ben Gardiner <bengardiner@nanometrics.ca>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>