lib/hexdump.c: return -EINVAL in case of error in hex2bin()

In some cases caller would like to use error code directly without
shadowing.

-EINVAL feels a rightful code to return in case of error in hex2bin().

Link: http://lkml.kernel.org/r/20170731135510.68023-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>