lib: Drop unnecessary check for hex digit

If we see 0x then we can assume this is the start of a hex value. It
does not seem necessary to check for a hex digit after that since it will
happen when parsing the value anyway.

Drop this check to simplify the code and reduce size. Add a few more test
cases for when a 0x prefix is used.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/str_ut.c b/test/str_ut.c
index 19f2c12..8133b21 100644
--- a/test/str_ut.c
+++ b/test/str_ut.c
@@ -84,6 +84,8 @@
 		/* Base 10 and base 16 */
 		ut_assertok(run_strtoul(uts, str2, 10, 1099, 4, upper));
 		ut_assertok(run_strtoul(uts, str2, 16, 0x1099ab, 6, upper));
+		ut_assertok(run_strtoul(uts, str3, 16, 0xb, 3, upper));
+		ut_assertok(run_strtoul(uts, str3, 10, 0, 1, upper));
 
 		/* Invalid string */
 		ut_assertok(run_strtoul(uts, str1, 10, 0, 0, upper));