rsi: Fixed errors and warnings reported by static code analyzers.

Fixed a potential buffer overflow in 'rsi_rates' and a sparse warning
related to difference in endianness in rsi_91x_mgmt.c.

Signed-off-by: Jahnavi Meher <jahnavi.meher@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>