m25pe80 dataflash: put 50 usec between WREN and PW instructions

I'm using a m25pe80 8Mbit spi dataflash. The PW (Page write) instruction is 
preceded by a WREN instruction but be careful to insert a delay between WREN 
and PW otherwise some page couldn't  be written. I hope could help someone.

Bye,
Antonio.