fbdev: Declare src parameter of fb_pad_ helpers as constant

fbdev: Declare src parameter of fb_pad_ helpers as constant

Fbdev's padding helpers do not modify the source buffer. Declare the
parameter as 'const'.

Fbcon's font-rendering code calls these helpers with the font data.
Declaring src as const will allow for making the font data constant
as well.

While at it, also remove the extern qualifier from the function
declarations in the header file.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Helge Deller <deller@gmx.de>