net: sfp: remove unused ctype.h include

An include of linux/ctype.h was added in commit 1323061a
("net: phy: sfp: Add HWMON support for module sensors") but nothing
was used from this header file. Remove this unnecessary include.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>