greybus: i2c: fix name conflict between function and struct: gb_i2c_transfer_request

'gb_i2c_transfer_request' is the name given to a function and a struct. Though
we don't get any compilation errors/warnings about it, but the names should be
unique.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>