staging: fbtft: improve a syslog message in fbtft_register_framebuffer

There are drivers like fb_sh1106 which allocate a txbuf less than 1 KiB.
In this case the message would include "0 KiB buffer memory" what
doesn't make sense and in general I doubt that there is any benefit in
informing the user about allocation of a very small buffer.

Therefore omit this message part if the buffer has less than 1 KiB.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>