staging: fbtft: Replace magic number with constant

Current call to strncmp() uses a magic number. There is a compile
time constant defined for this buffer, included and used already at
other sites in the file.

Remove magic number. Replace with pre-existing compile time constant.

Signed-off-by: Tobin C. Harding <me@tobin.cc>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>