rsi: rsi_91x_ps: remove redundant code in str_psstate

"INVALID_STATE" is already being returned in the default case and this
code cannot be reached.

Addresses-Coverity-ID: 1398384
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>