libpci: ecam: Fix discards const from pointer target

libpci: ecam: Fix discards const from pointer target

endptr is used as the return from strchr(addrs, ',') which is a const
char. endptr is subsequently used as a pointer to the token which is
char. Fix by not reusing endptr, but declare addrsptr as a const char *
pointer for use in the first case addressing the warning.

fixes:
    ecam.c: In function 'parse_next_addrs':
    ecam.c:620:10: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
      620 |   endptr = strchr(addrs, ',');
          |          ^

Signed-off-by: Rudi Heitbaum <rudi@heitbaum.com>