env: sf: fix environment in SPI NOR

commit 9a9d66f5 ("env: add spi_flash_read_env function")

breaks Environment functionality, as it reads only
until 2 \0 are found, but fills the buffer with 0x0
instead 0xff which leads in an incorrect crc sum.

Fix: init the read buffer with 0xff instead 0x00

Signed-off-by: Heiko Schocher <hs@denx.de>