tty: mips_ejtag_fdc: fix one more u8 warning

tty: mips_ejtag_fdc: fix one more u8 warning

The LKP robot complains about:
   drivers/tty/mips_ejtag_fdc.c:1224:31: error: incompatible pointer types passing 'const char *[1]' to parameter of type 'const u8 **' (aka 'const unsigned char **')

Fix this by turning the missing pieces (fetch from kgdbfdc_wbuf) to u8
too. Note the filling part (kgdbfdc_write_char()) already receives and
stores u8 to kgdbfdc_wbuf.

Fixes: ce7cbd9a6c81 ("tty: mips_ejtag_fdc: use u8 for character pointers")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202501101327.oGdWbmuk-lkp@intel.com/
Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20250110115228.603980-1-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>