rtl8xxxu: rtl8192eu: Map out EFUSE TX power area

TX power values are laid out differently in EFUSE found in RTL8192EU &
RTL8188EU devices.  TX power indices and differences for each RF path
are not interleaved (A, B, A, B), as in other chips, but follow one
another (A, B, C, D).

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>