tty: mips_ejtag_fdc: Fix passing incompatible pointer type warning

mips_ejtag_fdc_encode() method expects having a first argument passed of
the "u8 **" type, meanwhile the driver passes the "const char **" type.
That causes the next build-warning:

drivers/tty/mips_ejtag_fdc.c: In function ‘mips_ejtag_fdc_console_write’:
drivers/tty/mips_ejtag_fdc.c:343:32: error: passing argument 1 of ‘mips_ejtag_fdc_encode’ from incompatible pointer type [-Werror=incompatible-pointer-types]
   word = mips_ejtag_fdc_encode(&buf_ptr, &buf_len, 1);
                                ^
drivers/tty/mips_ejtag_fdc.c:216:24: note: expected ‘const u8 ** {aka const unsigned char **}’ but argument is of type ‘const char **’
 static struct fdc_word mips_ejtag_fdc_encode(const u8 **ptrs,
                        ^~~~~~~~~~~~~~~~~~~~~

Fix it by altering the type of the pointer which is passed to the
mips_ejtag_fdc_encode() method.

Fixes: ce7cbd9a ("tty: mips_ejtag_fdc: use u8 for character pointers")
Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>