cmd_sf: add size checking to spi flash commands

cmd_sf: add size checking to spi flash commands

SPI flash operations inadvertently stretching beyond the flash size will
result in a wraparound. This may be particularly dangerous when burning
u-boot, because the flash contents will be corrupted rendering the board
unusable, without any warning being issued.
So add a consistency checking so not to overflow past the flash size.

Signed-off-by: Gerlando Falauto <gerlando.falauto@keymile.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>