parisc/sticore: Avoid hard-coding built-in font charcount

sti_select_fbfont() and sti_cook_fonts() are hard-coding the number of
characters of our built-in fonts as 256. Recently, we included that
information in our kernel font descriptor `struct font_desc`, so use
`fbfont->charcount` instead of hard-coded values.

Depends on patch "Fonts: Add charcount field to font_desc".

Signed-off-by: Peilin Ye <yepeilin.cs@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/037186fb50cf3d17bb7bc9482357635b9df6076e.1605169912.git.yepeilin.cs@gmail.com