iwlagn: fix firmware loading TLV error path

gcc complains about the firmware loading:

iwl-agn.c: In function ‘iwlagn_load_firmware’:
iwl-agn.c:1860: warning: ‘tlv_len’ may be used uninitialized in this function
iwl-agn.c:1861: warning: ‘tlv_type’ may be used uninitialized in this function
iwl-agn.c:1862: warning: ‘tlv_data’ may be used uninitialized in this function

This is almost correct but we do do break out of the TLV
parsing loop when setting ret. However, the code is hard
to follow, and clearly even the compiler is having issues
with it too.

Additionally, however, the current code is wrong. If there
is a TLV length check error, the code will report
	invalid TLV after parsing: ...
because "len" will still be non-zero as we broke out of
the loop.

So to remove the warning and fix that issue, make the code
easier to read by doing length checking with an error label.
As a result, we can completely remove the "ret" variable.

Also, while at it, remove the "fixed_tlv_size" variable
since each TLV type has its own specified length, it just
happens that we have only variable length, flags (0 length)
and u32 TLVs right now. It should still be checked with more
explicit length checks to make it easier to understand.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>