target/hexagon/idef-parser: Use gen_tmp for LPCFG

The GET_USR_FIELD macro initializes the output, so the initial assignment
of zero is discarded.  This is the only use of get_tmp_value outside of
parser-helper.c, so make it static.

Reviewed-by: Taylor Simpson <tsimpson@quicinc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

target/hexagon/idef-parser/parser-helpers.h

@@ -154,12 +154,6 @@ HexValue gen_tmp(Context *c,
                  unsigned bit_width,
                  HexSignedness signedness);
 
-HexValue gen_tmp_value(Context *c,
-                       YYLTYPE *locp,
-                       const char *value,
-                       unsigned bit_width,
-                       HexSignedness signedness);
-
 HexValue gen_imm_value(Context *c __attribute__((unused)),
                        YYLTYPE *locp,
                        int value,