firmware: tegra: fix strncpy()/strncat() confusion

firmware: tegra: fix strncpy()/strncat() confusion

The way that bpmp_populate_debugfs_inband() uses strncpy()
and strncat() makes no sense since the size argument for
the first is insufficient to contain the trailing '/'
and the second passes the length of the input rather than
the output, which triggers a warning:

In function 'strncat',
    inlined from 'bpmp_populate_debugfs_inband' at ../drivers/firmware/tegra/bpmp-debugfs.c:422:4:
include/linux/string.h:289:30: warning: '__builtin_strncat' specified bound depends on the length of the source argument [-Wstringop-overflow=]
  289 | #define __underlying_strncat __builtin_strncat
      |                              ^
include/linux/string.h:367:10: note: in expansion of macro '__underlying_strncat'
  367 |   return __underlying_strncat(p, q, count);
      |          ^~~~~~~~~~~~~~~~~~~~
drivers/firmware/tegra/bpmp-debugfs.c: In function 'bpmp_populate_debugfs_inband':
include/linux/string.h:288:29: note: length computed here
  288 | #define __underlying_strlen __builtin_strlen
      |                             ^
include/linux/string.h:321:10: note: in expansion of macro '__underlying_strlen'
  321 |   return __underlying_strlen(p);

Simplify this to use an snprintf() instead.

Fixes: 5e37b9c137ee ("firmware: tegra: Add support for in-band debug")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>