mtd: cfi_cmdset_0002: Invalidate cache after entering/exiting OTP memory

When the one time programmable (OTP) memory region is entered by
issuing the 0xaa/0x55/0x88 command, the OTP memory occupies the
addresses which are normally used by the first sector of the regular
flash memory. This patch therefore invalidates cache for this
addresses after entering/exiting OTP memory.

This patch also moves the code into separate functions.

Signed-off-by: Christian Riesch <christian.riesch@omicron.at>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>