powerpc/espi: remove write command length check

Current espi controller driver assumes the command length of write command is
not equal to '1', it was made based on SPANSION SPI flash, but some SPI flash
driver such as SST does use write command length as '1', so write command on
SST SPI flash will not work. And the length check for write command is not
necessary for SPANSION, though it's harmless for SPANSION, it will stop write
operation on flashes like SST, so we remove the check.

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>