cfi_flash: reset timer in flash status check

This patch adds reset_timer() before the flash status check
waiting loop.

Since the timer is basically running asynchronous to the cfi
code, it is possible to call get_timer(0), then only a few
_SYSCLK_ cycles later an interrupt is generated. This causes
timeout even though much less time has elapsed. So the timer
period registers should be reset before get_timer(0) is
called.

There is similar usage in nand_base.c.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Signed-off-by: Stefan Roese <sr@denx.de>