Busy waiting in w25qxxxjv_erase_sector() was without nxsig_usleep and
was causing the entire system to freeze for significant amount of time
as sector erase takes some time.
This commit adds nxsig_usleep into busy waiting to prevent system lock.
Sleep is set to sector erase time based on W25Q series datasheet.
Signed-off-by: Michal Lenc <michallenc@seznam.cz>