vt: fix kernel-doc warnings in ucs_get_fallback()

vt: fix kernel-doc warnings in ucs_get_fallback()

Use the correct function parameter name in ucs_get_fallback() to prevent
kernel-doc warnings:

Warning: drivers/tty/vt/ucs.c:218 function parameter 'cp' not described in 'ucs_get_fallback'
Warning: drivers/tty/vt/ucs.c:218 Excess function parameter 'base' description in 'ucs_get_fallback'

Fixes: fe26933cf1e1 ("vt: add ucs_get_fallback()")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Nicolas Pitre <npitre@baylibre.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: linux-serial@vger.kernel.org
Reviewed-by: Nicolas Pitre <npitre@baylibre.com>.
Link: https://lore.kernel.org/r/20250611020229.2650595-1-rdunlap@infradead.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>