tg3: Eliminate tg3_nvram_read_swab()

The remaining uses of tg3_nvram_read_swab() either intended to read the
data from NVRAM exactly as tg3_nvram_read_be32() did or hide deeper
interpretations of the data.  For the former case, a direct replacement
of tg3_nvram_read_swab() with tg3_nvram_read_be32() is in order.  For
the latter case, we remove tg3_nvram_read_swab() and document what the
code is really doing.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>