psplash: remove unused parameter length from parse_command

This fixes following unused-parameter warning:

psplash.c: In function 'parse_command':
psplash.c:105:49: warning: unused parameter 'length'
[-Wunused-parameter]

Signed-off-by: Richard Leitner <richard.leitner@skidata.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>