env_ubi.c: Correct pointer error in env load

The variable "buf" in this function is a char array, and the
function ubi_volume_read is expecting a char *.  In the call, the
address of the pointer is being taken, incorrectly passing a
char **.  The compiler warning was being silenced by the cast.
Remove the address operator and the cast.

Signed-off-by: Kevin Smith <kevin.smith@elecsyscorp.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Tom Rini <trini@konsulko.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Heiko Schocher <hs@denx.de>