mtd: parsers: trx: fix pr_err format for printing offset

This fixes following warning:
include/linux/kern_levels.h:4:18: warning: format '%X' expects argument of type 'unsigned int', but argument 2 has type 'size_t {aka long unsigned int}' [-Wformat=]

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>