spi: sh_qspi: Replace ad hoc waiting with wait_for_bit

Replace the ad-hoc endless loops with wait_for_bit() with
reasonable timeout. Note that the loops had internal 10uS
delays, although there is no reason for those on this HW,
so they are dropped.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>