sata: namespace curr_device variable

The curr_device variable really should be namespaced with a "sata_" prefix
since it is only used by the sata code.  It also avoids random conflicts
with other pieces of code (like cmd_mmc):
common/libcommon.a(cmd_sata.o):(.data.curr_device+0x0):
	multiple definition of `curr_device'
common/libcommon.a(cmd_mmc.o):(.data.curr_device+0x0): first defined here

Signed-off-by: Mike Frysinger <vapier@gentoo.org>