drivers: soc: fwlog: Use max_t() to fix the comparison warning

We definitely need a cast for this comparison, so use max_t() to fix
the following build warning.
include/linux/minmax.h:20:35: warning: comparison of distinct pointer types lacks a cast
   20 |         (!!(sizeof((typeof(x) *)1 == (typeof(y) *)1)))
      |                                   ^~
drivers/soc/marvell/fwlog.c:61:33: note: in expansion of macro 'max'
   61 |                                 max(rdbuf - fwlog_buf, *ppos);
      |                                 ^~~

Signed-off-by: Kevin Hao <kexin.hao@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>