realtek_mst_i2c_spi.c: Fix _spi_write256() as documented

Turns out broken erasures highlighted some of the issues
in the write256 implementation. After a fair amount of
time deciphering scarce documentation details a correct
implementation was finally derived.

V.2: Rename 'start_program() -> execute_write()' to
  clarify the intention and not to overload the term
  'program' since the MST actually runs a 'program'
  itself.

BUG=b:152558985,b:148745673
BRANCH=none
TEST=flashrom -p realtek_mst_i2c_spi:bus=8 -E &&
  flashrom -p realtek_mst_i2c_spi:bus=8 -w foo &&
  flashrom -p realtek_mst_i2c_spi:bus=8 -r foo &&
  hexdump -C foo

Change-Id: If61ff95697f886d3301a907b76283322c39ef5c7
Signed-off-by: Edward O'Callaghan <quasisec@google.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/41080


Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Sam McNally <sammc@google.com>