ACPICA: acpidump: Fix code issue in invoking fread in the loop.

This patch fixes an issue that the while loop is not needed as fread()
should return exact the bytes of expected.

The patch is tested by runing diff against the output of "-c" mode and
the normal mode, and only finds the following differences:
1. table addresses: the "-c" mode will always fill 0x0000000000000000 for
                    the address.
2. RSDP/RSDT/XSDT: there is no generation of such tables for "-c" mode.
So the test result shows the fix is valid.  Lv Zheng.

Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>