hw/drivers/spiflash: Fix write over page boundary
hw/drivers/spiflash: Fix write over page boundary

A typo... we were using invalid buffer pointer so each write operation
would write from the beginning of input buffer.

mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core