rtlwifi: rtl8723be: Fix array dimension problems

Commit a619d1ab leads to the following static checker warning:

drivers/net/wireless/rtlwifi/rtl8723be/phy.c:667 _rtl8723be_store_tx_power_by_rate()
error: buffer overflow 'rtlphy->tx_power_by_rate_offset[band]' 4 <= 5

This warning arises because the code is testing the indices for the wrong maximum
values. In addition, the tests merely putput a warning, and then procedes to
corrupt memory. With this change, any such invalid memory access is avoided.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>