eth: fbnic: fix ubsan complaints about OOB accesses

eth: fbnic: fix ubsan complaints about OOB accesses

UBSAN complains that we reach beyond the end of the log entry:

   UBSAN: array-index-out-of-bounds in drivers/net/ethernet/meta/fbnic/fbnic_fw_log.c:94:50
   index 71 is out of range for type 'char [*]'
   Call Trace:
    <TASK>
    ubsan_epilogue+0x5/0x2b
    fbnic_fw_log_write+0x120/0x960
    fbnic_fw_parse_logs+0x161/0x210

We're just taking the address of the character after the array,
so this really seems like something that should be legal.
But whatever, easy enough to silence by doing direct pointer math.

Fixes: c2b93d6beca8 ("eth: fbnic: Create ring buffer for firmware logs")
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20250709205910.3107691-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>