Fix a few gcc warnings.

Noticed while building all of mpc8xx. Also
constify usage string in timer.c
Warnings fixed are:
timer.c: In function 'timer':
timer.c:189: warning: format not a string literal and no format arguments
timer.c:258: warning: format not a string literal and no format arguments
atm.c: In function 'atmUnload':
atm.c:99: warning: array subscript is above array bounds
atm.c: In function 'atmLoad':
atm.c:65: warning: array subscript is above array bounds
codec.c: In function 'codsp_write_pop_int':
codec.c:678: warning: array subscript is above array bounds
codec.c: In function 'codsp_write_cop_short':
codec.c:585: warning: array subscript is above array bounds
codec.c: In function 'codsp_write_sop_int':
codec.c:512: warning: array subscript is above array bounds

Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>