binman: Remove additional backslash

The origin patch didn't have this change and it was caused by manual
resolution where additional backslash was added.

Fixes: 6723b4c6 ("binman: Call helper function binman_set_rom_offset() to fill offset")
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>