wireless: airo: rename 'register' variable

'register' is a keyword in C and cannot be used in place of a
variable name, as shown by this -Wextra warning:

drivers/net/wireless/cisco/airo.c:1105:29: error: 'register' is not at beginning of declaration [-Werror=old-style-declaration]

This replaces the 'register' keyword with a 'reg' identifier in
the declaration, which matches the definition and has the intended
meaning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>