wt_icache: don't fallback to the TLB_MISS state when stalled in READ

Currently, when there is an I$ miss but the icache is locked in the READ
state, it would go in the TLB_MISS in the next cycle, which is useless.

This fixes this by setting the next state to READ when `stall_req_i' is
set in the case of a cache miss.

Signed-off-by: Alban Gruin <alban.gruin@irit.fr>