common/cmd_fdc.c: Fix GCC 4.6 build warnings

Fix:
cmd_fdc.c: In function 'fdc_read_data':
cmd_fdc.c:435:6: warning: variable 'flags' set but not used
[-Wunused-but-set-variable]
cmd_fdc.c:432:16: warning: variable 'pcn' set but not used
[-Wunused-but-set-variable]
cmd_fdc.c:431:20: warning: variable 'lastblk' set but not used
[-Wunused-but-set-variable]

Note: no attempts were made to otherwise cleanup the code.

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