Fix a bug in the slave serial programming mode for the Xilinx

Spartan2/3 FPGAs. The old code used "< 0" on a "char" type to test if
the most significant bit was set, which did not work on any
architecture where "char" defaulted to be an unsigned type.

Based on a patch by Angelos Manousaridis <amanous@inaccessnetworks.com>

Signed-off-by: Wolfgang Denk <wd@denx.de>