staging:rtl8192u: Rename HW90_BLOCK_E - Style

Rename the enumerated type HW90_BLOCK_E to hw90_block_e. Whilst this is
not flagged by checkpatch types are meant to be in lowercase.

This is a coding style change which should have no impact on runtime
code execution.

Signed-off-by: John Whitmore <johnfwhitmore@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>