soc: cn10k-fwlog: fix compile warning for cn10k-fwlog driver

commit 965e7bf2

(drivers: soc: cn10k: Add support for retrieving
firmware boot logs) introduced incompatible comparison and useless
variables, this will cause compile warning below. Remove unused variable
and add cast for incompatible type to fix it.

drivers/soc/marvell/cn10k-fwlog.c: In function 'fwlogs_read':
drivers/soc/marvell/cn10k-fwlog.c:58:60: warning: comparison between pointer and integer
   58 |                 if ((char *)(fwlog_hdr->fwlog_ptr + *ppos) == fwlog_hdr->fwlog_end)
      |                                                            ^~
drivers/soc/marvell/cn10k-fwlog.c:38:17: warning: unused variable 'ret' [-Wunused-variable]
   38 |         ssize_t ret = 0;
      |                 ^~~

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