rtl8xxxu: Don't check for illegal offset when reading from efuse

It is enough to check for either illegal offset or illegal map address
because map address is a value derived from an offset:

  map_addr = offset * 8
  EFUSE_MAP_LEN = EFUSE_MAX_SECTION_8723A * 8

Leave just the check for an illegal map address because its upper
bound (EFUSE_MAP_LEN) is used also in a couple other places.

Signed-off-by: Jakub Sitnicki <jsitnicki@gmail.com>
Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>