[plustek] Add 1ms delay before writing to register 0x59 during reseting registers

Hi,

this merge request fixes issue #137 (closed) - the cause of the issue is a device internal problem - a specific register - 0x59 - is not capable of quick bulk transfers as XHCI USB controller does, and writing to it ends with errno EPROTO. The fix simulates EHCI USB controller behavior before writing to the register, which means it sleeps for 1ms, and then issues a transfer to the register. This behavior can be turned on via SANE_PLUSTEK_USB_WORKAROUND environmental variable.

It is tested by reporter in Fedora bug and it works.

Would you mind merging it to the project? Please tell me if I should change something.

Have a nice day,

Zdenek