spl: dfu: Fix printed variable name

The SPL DFU uses dfu_alt_info_N variable name to determine the DFU
configuration, where N is the name of the media (e.g. ram). It does
not use the plain dfu_alt_info. Print the name of the missing env
variable in case of a failure instead of printing dfu_alt_info,
which is just the name of the parameter passed to spl_dfu_cmd().

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Tom Rini <trini@konsulko.com>
1 file changed