arm: socfpga: agilex5: Coverity issue cleanup in jtag_usercode check

arm: socfpga: agilex5: Coverity issue cleanup in jtag_usercode check

A redundant comparison (jtag_usercode >= 0) was removed from the
condition checking jtag_usercode. Since jtag_usercode is an unsigned
integer, the check for non-negativity was always true. The code was
simplified to resolve the Coverity warning.

Signed-off-by: Tingting Meng <tingting.meng@altera.com>
Reviewed-by: Tien Fong Chee <tien.fong.chee@intel.com>